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Message-Id: <200705291415.31970.rjw@sisk.pl>
Date: Tue, 29 May 2007 14:15:30 +0200
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: Pavel Machek <pavel@....cz>
Cc: LKML <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Gautham R Shenoy <ego@...ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Nigel Cunningham <nigel@...el.suspend2.net>,
Oleg Nesterov <oleg@...sign.ru>
Subject: Re: [RFC][PATCH][EXPERIMENTAL] Make kernel threads nonfreezable by default
Hi,
On Tuesday, 29 May 2007 13:31, Pavel Machek wrote:
> Hi!
>
> > > > Index: linux-2.6.22-rc3/Documentation/power/kernel_threads.txt
> > > > ===================================================================
> > > > --- linux-2.6.22-rc3.orig/Documentation/power/kernel_threads.txt
> > > > +++ linux-2.6.22-rc3/Documentation/power/kernel_threads.txt
> > > > @@ -1,13 +1,22 @@
> > > > -KERNEL THREADS
> > > > +The Freezer and Kernel Threads
> > > >
> > > > -
> > > > -Freezer
> > > > -
> > > > -Upon entering a suspended state the system will freeze all
> > > > -tasks. This is done by delivering pseudosignals. This affects
> > > > -kernel threads, too. To successfully freeze a kernel thread
> > > > -the thread has to check for the pseudosignal and enter the
> > > > -refrigerator. Code to do this looks like this:
> > > > +Before entering a system-wide suspend state as well as before creating a
> > > > +hibernation snapshot image the system will freeze all tasks, which is done
> > > > +by delivering fake signals. This affects kernel threads too, but they won't be
> > > > +frozen unless they declare that they want to. For this purpose
> > >
> > > Well... and unless thread that does disk writes or DMA _wants_ to, you
> > > have nice disk corruption... It should be pointed out that it is not
> > > voluntary for those types of threads.
> >
> > Well, that really depends.
> >
> > I think the only thing that must not be written to disk is filesystem stuff.
> > The other things (eg. writing directly to block devices etc.) doesn't hurt us.
>
> Well.. it can write anywhere it wants (filesystem or not) as long as
> the system is not going to be confused after resume by its caches not
> matching on-disk state. I'd prefer it not to write anywhere at all.
OK
Please have a look at the current version of the patch (appended).
I have followed the Nigel's suggestion not to change the current behavior
in this patch (I'll add a couple of patches removing the freezability from
some kernel threads), with one exception: I couldn't figure out any reason
to have try_to_freeze() called in net/sunrpc/svcsock.c:svc_recv() .
I've also added a piece of documentation, freezing-of-tasks.txt . Please
see if it's not missing anything (I'd like it to be quite complete).
Greetings,
Rafael
Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
---
Documentation/power/freezing-of-tasks.txt | 160 ++++++++++++++++++++++++++++++
Documentation/power/kernel_threads.txt | 40 -------
Documentation/power/swsusp.txt | 18 ---
arch/i386/kernel/apm.c | 1
arch/i386/kernel/io_apic.c | 1
drivers/block/loop.c | 7 -
drivers/block/pktcdvd.c | 1
drivers/char/apm-emulation.c | 11 --
drivers/char/hvc_console.c | 1
drivers/edac/edac_mc.c | 1
drivers/ieee1394/ieee1394_core.c | 2
drivers/ieee1394/nodemgr.c | 1
drivers/input/gameport/gameport.c | 1
drivers/input/serio/serio.c | 1
drivers/input/touchscreen/ucb1400_ts.c | 1
drivers/macintosh/therm_adt746x.c | 1
drivers/macintosh/windfarm_core.c | 1
drivers/md/md.c | 1
drivers/media/dvb/dvb-core/dvb_frontend.c | 1
drivers/media/video/cx88/cx88-tvaudio.c | 1
drivers/media/video/msp3400-kthreads.c | 5
drivers/media/video/tvaudio.c | 2
drivers/media/video/video-buf-dvb.c | 1
drivers/media/video/vivi.c | 1
drivers/mfd/ucb1x00-ts.c | 1
drivers/mmc/card/queue.c | 6 -
drivers/mtd/mtd_blkdevs.c | 2
drivers/mtd/ubi/wl.c | 1
drivers/net/wireless/airo.c | 3
drivers/net/wireless/libertas/main.c | 1
drivers/pcmcia/cs.c | 1
drivers/pnp/pnpbios/core.c | 1
drivers/scsi/libsas/sas_scsi_host.c | 2
drivers/scsi/scsi_error.c | 2
drivers/usb/atm/ueagle-atm.c | 1
drivers/usb/core/hub.c | 1
drivers/usb/gadget/file_storage.c | 2
drivers/usb/storage/usb.c | 3
drivers/video/ps3fb.c | 1
drivers/w1/w1.c | 1
fs/cifs/cifsfs.c | 1
fs/cifs/connect.c | 1
fs/jffs2/background.c | 1
fs/lockd/svc.c | 2
fs/xfs/linux-2.6/xfs_super.c | 1
include/linux/freezer.h | 9 +
init/do_mounts_initrd.c | 7 -
kernel/audit.c | 1
kernel/exit.c | 5
kernel/fork.c | 2
kernel/rcutorture.c | 3
kernel/rtmutex-tester.c | 1
kernel/sched.c | 2
kernel/softirq.c | 1
kernel/softlockup.c | 1
kernel/workqueue.c | 4
mm/pdflush.c | 1
mm/vmscan.c | 1
net/bluetooth/bnep/core.c | 1
net/bluetooth/cmtp/core.c | 1
net/bluetooth/hidp/core.c | 1
net/bluetooth/rfcomm/core.c | 1
net/core/pktgen.c | 2
net/sunrpc/svcsock.c | 3
64 files changed, 230 insertions(+), 113 deletions(-)
Index: linux-2.6.22-rc3/kernel/exit.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/exit.c
+++ linux-2.6.22-rc3/kernel/exit.c
@@ -389,6 +389,11 @@ void daemonize(const char *name, ...)
* they would be locked into memory.
*/
exit_mm(current);
+ /*
+ * We don't want to have TIF_FREEZE set if the system-wide hibernation
+ * or suspend transition begins right now.
+ */
+ current->flags |= PF_NOFREEZE;
set_special_pids(1, 1);
proc_clear_tty(current);
Index: linux-2.6.22-rc3/include/linux/freezer.h
===================================================================
--- linux-2.6.22-rc3.orig/include/linux/freezer.h
+++ linux-2.6.22-rc3/include/linux/freezer.h
@@ -118,6 +118,14 @@ static inline int freezer_should_skip(st
return !!(p->flags & PF_FREEZER_SKIP);
}
+/*
+ * Tell the freezer that the current task should be frozen by it
+ */
+static inline void set_freezable(void)
+{
+ current->flags &= ~PF_NOFREEZE;
+}
+
#else
static inline int frozen(struct task_struct *p) { return 0; }
static inline int freezing(struct task_struct *p) { return 0; }
@@ -134,6 +142,7 @@ static inline int try_to_freeze(void) {
static inline void freezer_do_not_count(void) {}
static inline void freezer_count(void) {}
static inline int freezer_should_skip(struct task_struct *p) { return 0; }
+static inline void set_freezable(void) {}
#endif
#endif /* LINUX_FREEZER_H */
Index: linux-2.6.22-rc3/kernel/fork.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/fork.c
+++ linux-2.6.22-rc3/kernel/fork.c
@@ -920,7 +920,7 @@ static inline void copy_flags(unsigned l
{
unsigned long new_flags = p->flags;
- new_flags &= ~(PF_SUPERPRIV | PF_NOFREEZE);
+ new_flags &= ~PF_SUPERPRIV;
new_flags |= PF_FORKNOEXEC;
if (!(clone_flags & CLONE_PTRACE))
p->ptrace = 0;
Index: linux-2.6.22-rc3/arch/i386/kernel/io_apic.c
===================================================================
--- linux-2.6.22-rc3.orig/arch/i386/kernel/io_apic.c
+++ linux-2.6.22-rc3/arch/i386/kernel/io_apic.c
@@ -667,6 +667,7 @@ static int balanced_irq(void *unused)
set_pending_irq(i, cpumask_of_cpu(0));
}
+ set_freezable();
for ( ; ; ) {
time_remaining = schedule_timeout_interruptible(time_remaining);
try_to_freeze();
Index: linux-2.6.22-rc3/Documentation/power/kernel_threads.txt
===================================================================
--- linux-2.6.22-rc3.orig/Documentation/power/kernel_threads.txt
+++ /dev/null
@@ -1,40 +0,0 @@
-KERNEL THREADS
-
-
-Freezer
-
-Upon entering a suspended state the system will freeze all
-tasks. This is done by delivering pseudosignals. This affects
-kernel threads, too. To successfully freeze a kernel thread
-the thread has to check for the pseudosignal and enter the
-refrigerator. Code to do this looks like this:
-
- do {
- hub_events();
- wait_event_interruptible(khubd_wait, !list_empty(&hub_event_list));
- try_to_freeze();
- } while (!signal_pending(current));
-
-from drivers/usb/core/hub.c::hub_thread()
-
-
-The Unfreezable
-
-Some kernel threads however, must not be frozen. The kernel must
-be able to finish pending IO operations and later on be able to
-write the memory image to disk. Kernel threads needed to do IO
-must stay awake. Such threads must mark themselves unfreezable
-like this:
-
- /*
- * This thread doesn't need any user-level access,
- * so get rid of all our resources.
- */
- daemonize("usb-storage");
-
- current->flags |= PF_NOFREEZE;
-
-from drivers/usb/storage/usb.c::usb_stor_control_thread()
-
-Such drivers are themselves responsible for staying quiet during
-the actual snapshotting.
Index: linux-2.6.22-rc3/Documentation/power/swsusp.txt
===================================================================
--- linux-2.6.22-rc3.orig/Documentation/power/swsusp.txt
+++ linux-2.6.22-rc3/Documentation/power/swsusp.txt
@@ -140,21 +140,11 @@ should be sent to the mailing list avail
website, and not to the Linux Kernel Mailing List. We are working
toward merging suspend2 into the mainline kernel.
-Q: A kernel thread must voluntarily freeze itself (call 'refrigerator').
-I found some kernel threads that don't do it, and they don't freeze
-so the system can't sleep. Is this a known behavior?
-
-A: All such kernel threads need to be fixed, one by one. Select the
-place where the thread is safe to be frozen (no kernel semaphores
-should be held at that point and it must be safe to sleep there), and
-add:
-
- try_to_freeze();
-
-If the thread is needed for writing the image to storage, you should
-instead set the PF_NOFREEZE process flag when creating the thread (and
-be very careful).
+Q: What is the freezing of tasks and why are we using it?
+A: The freezing of tasks is a mechanism by which user space processes and some
+kernel threads are controlled during hibernation or system-wide suspend (on some
+architectures). See freezing-of-tasks.txt for details.
Q: What is the difference between "platform" and "shutdown"?
Index: linux-2.6.22-rc3/drivers/block/pktcdvd.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/block/pktcdvd.c
+++ linux-2.6.22-rc3/drivers/block/pktcdvd.c
@@ -1594,6 +1594,7 @@ static int kcdrwd(void *foobar)
long min_sleep_time, residue;
set_user_nice(current, -20);
+ set_freezable();
for (;;) {
DECLARE_WAITQUEUE(wait, current);
Index: linux-2.6.22-rc3/drivers/char/hvc_console.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/char/hvc_console.c
+++ linux-2.6.22-rc3/drivers/char/hvc_console.c
@@ -679,6 +679,7 @@ int khvcd(void *unused)
int poll_mask;
struct hvc_struct *hp;
+ set_freezable();
__set_current_state(TASK_RUNNING);
do {
poll_mask = 0;
Index: linux-2.6.22-rc3/drivers/edac/edac_mc.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/edac/edac_mc.c
+++ linux-2.6.22-rc3/drivers/edac/edac_mc.c
@@ -1906,6 +1906,7 @@ static void do_edac_check(void)
static int edac_kernel_thread(void *arg)
{
+ set_freezable();
while (!kthread_should_stop()) {
do_edac_check();
Index: linux-2.6.22-rc3/drivers/ieee1394/nodemgr.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/ieee1394/nodemgr.c
+++ linux-2.6.22-rc3/drivers/ieee1394/nodemgr.c
@@ -1664,6 +1664,7 @@ static int nodemgr_host_thread(void *__h
unsigned int g, generation = 0;
int i, reset_cycles = 0;
+ set_freezable();
/* Setup our device-model entries */
nodemgr_create_host_dev_files(host);
Index: linux-2.6.22-rc3/drivers/input/gameport/gameport.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/input/gameport/gameport.c
+++ linux-2.6.22-rc3/drivers/input/gameport/gameport.c
@@ -445,6 +445,7 @@ static struct gameport *gameport_get_pen
static int gameport_thread(void *nothing)
{
+ set_freezable();
do {
gameport_handle_event();
wait_event_interruptible(gameport_wait,
Index: linux-2.6.22-rc3/drivers/input/serio/serio.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/input/serio/serio.c
+++ linux-2.6.22-rc3/drivers/input/serio/serio.c
@@ -384,6 +384,7 @@ static struct serio *serio_get_pending_c
static int serio_thread(void *nothing)
{
+ set_freezable();
do {
serio_handle_event();
wait_event_interruptible(serio_wait,
Index: linux-2.6.22-rc3/drivers/macintosh/therm_adt746x.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/macintosh/therm_adt746x.c
+++ linux-2.6.22-rc3/drivers/macintosh/therm_adt746x.c
@@ -335,6 +335,7 @@ static int monitor_task(void *arg)
{
struct thermostat* th = arg;
+ set_freezable();
while(!kthread_should_stop()) {
try_to_freeze();
msleep_interruptible(2000);
Index: linux-2.6.22-rc3/drivers/macintosh/windfarm_core.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/macintosh/windfarm_core.c
+++ linux-2.6.22-rc3/drivers/macintosh/windfarm_core.c
@@ -92,6 +92,7 @@ static int wf_thread_func(void *data)
DBG("wf: thread started\n");
+ set_freezable();
while(!kthread_should_stop()) {
if (time_after_eq(jiffies, next)) {
wf_notify(WF_EVENT_TICK, NULL);
Index: linux-2.6.22-rc3/drivers/media/dvb/dvb-core/dvb_frontend.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/media/dvb/dvb-core/dvb_frontend.c
+++ linux-2.6.22-rc3/drivers/media/dvb/dvb-core/dvb_frontend.c
@@ -523,6 +523,7 @@ static int dvb_frontend_thread(void *dat
dvb_frontend_init(fe);
+ set_freezable();
while (1) {
up(&fepriv->sem); /* is locked when we enter the thread... */
restart:
Index: linux-2.6.22-rc3/drivers/media/video/cx88/cx88-tvaudio.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/media/video/cx88/cx88-tvaudio.c
+++ linux-2.6.22-rc3/drivers/media/video/cx88/cx88-tvaudio.c
@@ -906,6 +906,7 @@ int cx88_audio_thread(void *data)
u32 mode = 0;
dprintk("cx88: tvaudio thread started\n");
+ set_freezable();
for (;;) {
msleep_interruptible(1000);
if (kthread_should_stop())
Index: linux-2.6.22-rc3/drivers/media/video/msp3400-kthreads.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/media/video/msp3400-kthreads.c
+++ linux-2.6.22-rc3/drivers/media/video/msp3400-kthreads.c
@@ -468,6 +468,7 @@ int msp3400c_thread(void *data)
v4l_dbg(1, msp_debug, client, "msp3400 daemon started\n");
+ set_freezable();
for (;;) {
v4l_dbg(2, msp_debug, client, "msp3400 thread: sleep\n");
msp_sleep(state, -1);
@@ -646,7 +647,7 @@ int msp3410d_thread(void *data)
int val, i, std, count;
v4l_dbg(1, msp_debug, client, "msp3410 daemon started\n");
-
+ set_freezable();
for (;;) {
v4l_dbg(2, msp_debug, client, "msp3410 thread: sleep\n");
msp_sleep(state,-1);
@@ -940,7 +941,7 @@ int msp34xxg_thread(void *data)
int val, i;
v4l_dbg(1, msp_debug, client, "msp34xxg daemon started\n");
-
+ set_freezable();
for (;;) {
v4l_dbg(2, msp_debug, client, "msp34xxg thread: sleep\n");
msp_sleep(state, -1);
Index: linux-2.6.22-rc3/drivers/media/video/tvaudio.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/media/video/tvaudio.c
+++ linux-2.6.22-rc3/drivers/media/video/tvaudio.c
@@ -271,7 +271,7 @@ static int chip_thread(void *data)
struct CHIPDESC *desc = chiplist + chip->type;
v4l_dbg(1, debug, &chip->c, "%s: thread started\n", chip->c.name);
-
+ set_freezable();
for (;;) {
set_current_state(TASK_INTERRUPTIBLE);
if (!kthread_should_stop())
Index: linux-2.6.22-rc3/drivers/media/video/video-buf-dvb.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/media/video/video-buf-dvb.c
+++ linux-2.6.22-rc3/drivers/media/video/video-buf-dvb.c
@@ -47,6 +47,7 @@ static int videobuf_dvb_thread(void *dat
int err;
dprintk("dvb thread started\n");
+ set_freezable();
videobuf_read_start(&dvb->dvbq);
for (;;) {
Index: linux-2.6.22-rc3/drivers/media/video/vivi.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/media/video/vivi.c
+++ linux-2.6.22-rc3/drivers/media/video/vivi.c
@@ -538,6 +538,7 @@ static void vivi_sleep(struct vivi_dmaqu
dprintk(1,"%s dma_q=0x%08lx\n",__FUNCTION__,(unsigned long)dma_q);
+ set_freezable();
add_wait_queue(&dma_q->wq, &wait);
if (!kthread_should_stop()) {
dma_q->frame++;
Index: linux-2.6.22-rc3/drivers/mfd/ucb1x00-ts.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/mfd/ucb1x00-ts.c
+++ linux-2.6.22-rc3/drivers/mfd/ucb1x00-ts.c
@@ -209,6 +209,7 @@ static int ucb1x00_thread(void *_ts)
DECLARE_WAITQUEUE(wait, tsk);
int valid = 0;
+ set_freezable();
add_wait_queue(&ts->irq_wait, &wait);
while (!kthread_should_stop()) {
unsigned int x, y, p;
Index: linux-2.6.22-rc3/drivers/mtd/ubi/wl.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/mtd/ubi/wl.c
+++ linux-2.6.22-rc3/drivers/mtd/ubi/wl.c
@@ -1346,6 +1346,7 @@ static int ubi_thread(void *u)
ubi_msg("background thread \"%s\" started, PID %d",
ubi->bgt_name, current->pid);
+ set_freezable();
for (;;) {
int err;
Index: linux-2.6.22-rc3/drivers/net/wireless/airo.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/net/wireless/airo.c
+++ linux-2.6.22-rc3/drivers/net/wireless/airo.c
@@ -3078,7 +3078,8 @@ static int airo_thread(void *data) {
struct net_device *dev = data;
struct airo_info *ai = dev->priv;
int locked;
-
+
+ set_freezable();
while(1) {
/* make swsusp happy with our thread */
try_to_freeze();
Index: linux-2.6.22-rc3/drivers/net/wireless/libertas/main.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/net/wireless/libertas/main.c
+++ linux-2.6.22-rc3/drivers/net/wireless/libertas/main.c
@@ -718,6 +718,7 @@ static int wlan_service_main_thread(void
init_waitqueue_entry(&wait, current);
+ set_freezable();
for (;;) {
lbs_pr_debug(1, "main-thread 111: intcounter=%d "
"currenttxskb=%p dnld_sent=%d\n",
Index: linux-2.6.22-rc3/drivers/pcmcia/cs.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/pcmcia/cs.c
+++ linux-2.6.22-rc3/drivers/pcmcia/cs.c
@@ -651,6 +651,7 @@ static int pccardd(void *__skt)
add_wait_queue(&skt->thread_wait, &wait);
complete(&skt->thread_done);
+ set_freezable();
for (;;) {
unsigned long flags;
unsigned int events;
Index: linux-2.6.22-rc3/drivers/pnp/pnpbios/core.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/pnp/pnpbios/core.c
+++ linux-2.6.22-rc3/drivers/pnp/pnpbios/core.c
@@ -160,6 +160,7 @@ static int pnp_dock_thread(void * unused
{
static struct pnp_docking_station_info now;
int docked = -1, d = 0;
+ set_freezable();
while (!unloading)
{
int status;
Index: linux-2.6.22-rc3/drivers/input/touchscreen/ucb1400_ts.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/input/touchscreen/ucb1400_ts.c
+++ linux-2.6.22-rc3/drivers/input/touchscreen/ucb1400_ts.c
@@ -292,6 +292,7 @@ static int ucb1400_ts_thread(void *_ucb)
sched_setscheduler(tsk, SCHED_FIFO, ¶m);
+ set_freezable();
while (!kthread_should_stop()) {
unsigned int x, y, p;
long timeout;
Index: linux-2.6.22-rc3/drivers/usb/atm/ueagle-atm.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/usb/atm/ueagle-atm.c
+++ linux-2.6.22-rc3/drivers/usb/atm/ueagle-atm.c
@@ -1168,6 +1168,7 @@ static int uea_kthread(void *data)
struct uea_softc *sc = data;
int ret = -EAGAIN;
+ set_freezable();
uea_enters(INS_TO_USBDEV(sc));
while (!kthread_should_stop()) {
if (ret < 0 || sc->reset)
Index: linux-2.6.22-rc3/drivers/usb/core/hub.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/usb/core/hub.c
+++ linux-2.6.22-rc3/drivers/usb/core/hub.c
@@ -2799,6 +2799,7 @@ loop:
static int hub_thread(void *__unused)
{
+ set_freezable();
do {
hub_events();
wait_event_interruptible(khubd_wait,
Index: linux-2.6.22-rc3/drivers/usb/gadget/file_storage.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/usb/gadget/file_storage.c
+++ linux-2.6.22-rc3/drivers/usb/gadget/file_storage.c
@@ -3435,6 +3435,8 @@ static int fsg_main_thread(void *fsg_)
sigmask(SIGTERM) | sigmask(SIGKILL) |
sigmask(SIGUSR1));
sigprocmask(SIG_SETMASK, &fsg->thread_signal_mask, NULL);
+ /* Allow the thread to be frozen */
+ set_freezable();
/* Arrange for userspace references to be interpreted as kernel
* pointers. That way we can pass a kernel pointer to a routine
Index: linux-2.6.22-rc3/drivers/usb/storage/usb.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/usb/storage/usb.c
+++ linux-2.6.22-rc3/drivers/usb/storage/usb.c
@@ -301,8 +301,6 @@ static int usb_stor_control_thread(void
struct us_data *us = (struct us_data *)__us;
struct Scsi_Host *host = us_to_host(us);
- current->flags |= PF_NOFREEZE;
-
for(;;) {
US_DEBUGP("*** thread sleeping.\n");
if(down_interruptible(&us->sema))
@@ -909,6 +907,7 @@ static int usb_stor_scan_thread(void * _
printk(KERN_DEBUG
"usb-storage: device found at %d\n", us->pusb_dev->devnum);
+ set_freezable();
/* Wait for the timeout to expire or for a disconnect */
if (delay_use > 0) {
printk(KERN_DEBUG "usb-storage: waiting for device "
Index: linux-2.6.22-rc3/drivers/video/ps3fb.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/video/ps3fb.c
+++ linux-2.6.22-rc3/drivers/video/ps3fb.c
@@ -812,6 +812,7 @@ static int ps3fb_ioctl(struct fb_info *i
static int ps3fbd(void *arg)
{
+ set_freezable();
while (!kthread_should_stop()) {
try_to_freeze();
set_current_state(TASK_INTERRUPTIBLE);
Index: linux-2.6.22-rc3/drivers/w1/w1.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/w1/w1.c
+++ linux-2.6.22-rc3/drivers/w1/w1.c
@@ -801,6 +801,7 @@ static int w1_control(void *data)
struct w1_master *dev, *n;
int have_to_wait = 0;
+ set_freezable();
while (!kthread_should_stop() || have_to_wait) {
have_to_wait = 0;
Index: linux-2.6.22-rc3/fs/cifs/cifsfs.c
===================================================================
--- linux-2.6.22-rc3.orig/fs/cifs/cifsfs.c
+++ linux-2.6.22-rc3/fs/cifs/cifsfs.c
@@ -849,6 +849,7 @@ static int cifs_oplock_thread(void * dum
__u16 netfid;
int rc;
+ set_freezable();
do {
if (try_to_freeze())
continue;
Index: linux-2.6.22-rc3/fs/cifs/connect.c
===================================================================
--- linux-2.6.22-rc3.orig/fs/cifs/connect.c
+++ linux-2.6.22-rc3/fs/cifs/connect.c
@@ -363,6 +363,7 @@ cifs_demultiplex_thread(struct TCP_Serve
GFP_KERNEL);
}
+ set_freezable();
while (!kthread_should_stop()) {
if (try_to_freeze())
continue;
Index: linux-2.6.22-rc3/fs/jffs2/background.c
===================================================================
--- linux-2.6.22-rc3.orig/fs/jffs2/background.c
+++ linux-2.6.22-rc3/fs/jffs2/background.c
@@ -81,6 +81,7 @@ static int jffs2_garbage_collect_thread(
set_user_nice(current, 10);
+ set_freezable();
for (;;) {
allow_signal(SIGHUP);
Index: linux-2.6.22-rc3/fs/xfs/linux-2.6/xfs_super.c
===================================================================
--- linux-2.6.22-rc3.orig/fs/xfs/linux-2.6/xfs_super.c
+++ linux-2.6.22-rc3/fs/xfs/linux-2.6/xfs_super.c
@@ -561,6 +561,7 @@ xfssyncd(
bhv_vfs_sync_work_t *work, *n;
LIST_HEAD (tmp);
+ set_freezable();
timeleft = xfs_syncd_centisecs * msecs_to_jiffies(10);
for (;;) {
timeleft = schedule_timeout_interruptible(timeleft);
Index: linux-2.6.22-rc3/init/do_mounts_initrd.c
===================================================================
--- linux-2.6.22-rc3.orig/init/do_mounts_initrd.c
+++ linux-2.6.22-rc3/init/do_mounts_initrd.c
@@ -56,12 +56,9 @@ static void __init handle_initrd(void)
sys_chroot(".");
pid = kernel_thread(do_linuxrc, "/linuxrc", SIGCHLD);
- if (pid > 0) {
- while (pid != sys_wait4(-1, NULL, 0, NULL)) {
- try_to_freeze();
+ if (pid > 0)
+ while (pid != sys_wait4(-1, NULL, 0, NULL))
yield();
- }
- }
/* move initrd to rootfs' /old */
sys_fchdir(old_fd);
Index: linux-2.6.22-rc3/kernel/audit.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/audit.c
+++ linux-2.6.22-rc3/kernel/audit.c
@@ -391,6 +391,7 @@ static int kauditd_thread(void *dummy)
{
struct sk_buff *skb;
+ set_freezable();
while (!kthread_should_stop()) {
skb = skb_dequeue(&audit_skb_queue);
wake_up(&audit_backlog_wait);
Index: linux-2.6.22-rc3/kernel/rtmutex-tester.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/rtmutex-tester.c
+++ linux-2.6.22-rc3/kernel/rtmutex-tester.c
@@ -260,6 +260,7 @@ static int test_func(void *data)
int ret;
current->flags |= PF_MUTEX_TESTER;
+ set_freezable();
allow_signal(SIGHUP);
for(;;) {
Index: linux-2.6.22-rc3/kernel/sched.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/sched.c
+++ linux-2.6.22-rc3/kernel/sched.c
@@ -5157,6 +5157,7 @@ static int migration_thread(void *data)
BUG_ON(rq->migration_thread != current);
set_current_state(TASK_INTERRUPTIBLE);
+ set_freezable();
while (!kthread_should_stop()) {
struct migration_req *req;
struct list_head *head;
@@ -5396,7 +5397,6 @@ migration_call(struct notifier_block *nf
p = kthread_create(migration_thread, hcpu, "migration/%d",cpu);
if (IS_ERR(p))
return NOTIFY_BAD;
- p->flags |= PF_NOFREEZE;
kthread_bind(p, cpu);
/* Must be high prio: stop_machine expects to yield to it. */
rq = task_rq_lock(p, &flags);
Index: linux-2.6.22-rc3/kernel/workqueue.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/workqueue.c
+++ linux-2.6.22-rc3/kernel/workqueue.c
@@ -282,8 +282,8 @@ static int worker_thread(void *__cwq)
struct cpu_workqueue_struct *cwq = __cwq;
DEFINE_WAIT(wait);
- if (!cwq->wq->freezeable)
- current->flags |= PF_NOFREEZE;
+ if (cwq->wq->freezeable)
+ set_freezable();
set_user_nice(current, -5);
Index: linux-2.6.22-rc3/mm/pdflush.c
===================================================================
--- linux-2.6.22-rc3.orig/mm/pdflush.c
+++ linux-2.6.22-rc3/mm/pdflush.c
@@ -92,6 +92,7 @@ struct pdflush_work {
static int __pdflush(struct pdflush_work *my_work)
{
current->flags |= PF_FLUSHER | PF_SWAPWRITE;
+ set_freezable();
my_work->fn = NULL;
my_work->who = current;
INIT_LIST_HEAD(&my_work->list);
Index: linux-2.6.22-rc3/net/core/pktgen.c
===================================================================
--- linux-2.6.22-rc3.orig/net/core/pktgen.c
+++ linux-2.6.22-rc3/net/core/pktgen.c
@@ -3274,6 +3274,8 @@ static int pktgen_thread_worker(void *ar
set_current_state(TASK_INTERRUPTIBLE);
+ set_freezable();
+
while (!kthread_should_stop()) {
pkt_dev = next_to_run(t);
Index: linux-2.6.22-rc3/net/sunrpc/svcsock.c
===================================================================
--- linux-2.6.22-rc3.orig/net/sunrpc/svcsock.c
+++ linux-2.6.22-rc3/net/sunrpc/svcsock.c
@@ -1436,7 +1436,6 @@ svc_recv(struct svc_rqst *rqstp, long ti
arg->len = (pages-1)*PAGE_SIZE;
arg->tail[0].iov_len = 0;
- try_to_freeze();
cond_resched();
if (signalled())
return -EINTR;
@@ -1461,8 +1460,6 @@ svc_recv(struct svc_rqst *rqstp, long ti
schedule_timeout(timeout);
- try_to_freeze();
-
spin_lock_bh(&pool->sp_lock);
remove_wait_queue(&rqstp->rq_wait, &wait);
Index: linux-2.6.22-rc3/arch/i386/kernel/apm.c
===================================================================
--- linux-2.6.22-rc3.orig/arch/i386/kernel/apm.c
+++ linux-2.6.22-rc3/arch/i386/kernel/apm.c
@@ -2309,7 +2309,6 @@ static int __init apm_init(void)
remove_proc_entry("apm", NULL);
return err;
}
- kapmd_task->flags |= PF_NOFREEZE;
wake_up_process(kapmd_task);
if (num_online_cpus() > 1 && !smp ) {
Index: linux-2.6.22-rc3/drivers/block/loop.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/block/loop.c
+++ linux-2.6.22-rc3/drivers/block/loop.c
@@ -576,13 +576,6 @@ static int loop_thread(void *data)
struct loop_device *lo = data;
struct bio *bio;
- /*
- * loop can be used in an encrypted device,
- * hence, it mustn't be stopped at all
- * because it could be indirectly used during suspension
- */
- current->flags |= PF_NOFREEZE;
-
set_user_nice(current, -20);
while (!kthread_should_stop() || lo->lo_bio) {
Index: linux-2.6.22-rc3/drivers/char/apm-emulation.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/char/apm-emulation.c
+++ linux-2.6.22-rc3/drivers/char/apm-emulation.c
@@ -329,13 +329,8 @@ apm_ioctl(struct inode * inode, struct f
/*
* Wait for the suspend/resume to complete. If there
* are pending acknowledges, we wait here for them.
- *
- * Note: we need to ensure that the PM subsystem does
- * not kick us out of the wait when it suspends the
- * threads.
*/
flags = current->flags;
- current->flags |= PF_NOFREEZE;
wait_event(apm_suspend_waitqueue,
as->suspend_state == SUSPEND_DONE);
@@ -365,13 +360,8 @@ apm_ioctl(struct inode * inode, struct f
/*
* Wait for the suspend/resume to complete. If there
* are pending acknowledges, we wait here for them.
- *
- * Note: we need to ensure that the PM subsystem does
- * not kick us out of the wait when it suspends the
- * threads.
*/
flags = current->flags;
- current->flags |= PF_NOFREEZE;
wait_event_interruptible(apm_suspend_waitqueue,
as->suspend_state == SUSPEND_DONE);
@@ -598,7 +588,6 @@ static int __init apm_init(void)
kapmd_tsk = NULL;
return ret;
}
- kapmd_tsk->flags |= PF_NOFREEZE;
wake_up_process(kapmd_tsk);
#ifdef CONFIG_PROC_FS
Index: linux-2.6.22-rc3/drivers/ieee1394/ieee1394_core.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/ieee1394/ieee1394_core.c
+++ linux-2.6.22-rc3/drivers/ieee1394/ieee1394_core.c
@@ -1133,8 +1133,6 @@ static int hpsbpkt_thread(void *__hi)
struct list_head tmp;
int may_schedule;
- current->flags |= PF_NOFREEZE;
-
while (!kthread_should_stop()) {
INIT_LIST_HEAD(&tmp);
Index: linux-2.6.22-rc3/drivers/md/md.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/md/md.c
+++ linux-2.6.22-rc3/drivers/md/md.c
@@ -4642,7 +4642,6 @@ static int md_thread(void * arg)
* many dirty RAID5 blocks.
*/
- current->flags |= PF_NOFREEZE;
allow_signal(SIGKILL);
while (!kthread_should_stop()) {
Index: linux-2.6.22-rc3/drivers/mmc/card/queue.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/mmc/card/queue.c
+++ linux-2.6.22-rc3/drivers/mmc/card/queue.c
@@ -42,11 +42,7 @@ static int mmc_queue_thread(void *d)
struct mmc_queue *mq = d;
struct request_queue *q = mq->queue;
- /*
- * Set iothread to ensure that we aren't put to sleep by
- * the process freezing. We handle suspension ourselves.
- */
- current->flags |= PF_MEMALLOC|PF_NOFREEZE;
+ current->flags |= PF_MEMALLOC;
down(&mq->thread_sem);
do {
Index: linux-2.6.22-rc3/drivers/mtd/mtd_blkdevs.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/mtd/mtd_blkdevs.c
+++ linux-2.6.22-rc3/drivers/mtd/mtd_blkdevs.c
@@ -80,7 +80,7 @@ static int mtd_blktrans_thread(void *arg
struct request_queue *rq = tr->blkcore_priv->rq;
/* we might get involved when memory gets low, so use PF_MEMALLOC */
- current->flags |= PF_MEMALLOC | PF_NOFREEZE;
+ current->flags |= PF_MEMALLOC;
spin_lock_irq(rq->queue_lock);
while (!kthread_should_stop()) {
Index: linux-2.6.22-rc3/drivers/scsi/libsas/sas_scsi_host.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/scsi/libsas/sas_scsi_host.c
+++ linux-2.6.22-rc3/drivers/scsi/libsas/sas_scsi_host.c
@@ -868,8 +868,6 @@ static int sas_queue_thread(void *_sas_h
{
struct sas_ha_struct *sas_ha = _sas_ha;
- current->flags |= PF_NOFREEZE;
-
while (1) {
set_current_state(TASK_INTERRUPTIBLE);
schedule();
Index: linux-2.6.22-rc3/drivers/scsi/scsi_error.c
===================================================================
--- linux-2.6.22-rc3.orig/drivers/scsi/scsi_error.c
+++ linux-2.6.22-rc3/drivers/scsi/scsi_error.c
@@ -1536,8 +1536,6 @@ int scsi_error_handler(void *data)
{
struct Scsi_Host *shost = data;
- current->flags |= PF_NOFREEZE;
-
/*
* We use TASK_INTERRUPTIBLE so that the thread is not
* counted against the load average as a running process.
Index: linux-2.6.22-rc3/kernel/rcutorture.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/rcutorture.c
+++ linux-2.6.22-rc3/kernel/rcutorture.c
@@ -518,7 +518,6 @@ rcu_torture_writer(void *arg)
VERBOSE_PRINTK_STRING("rcu_torture_writer task started");
set_user_nice(current, 19);
- current->flags |= PF_NOFREEZE;
do {
schedule_timeout_uninterruptible(1);
@@ -558,7 +557,6 @@ rcu_torture_fakewriter(void *arg)
VERBOSE_PRINTK_STRING("rcu_torture_fakewriter task started");
set_user_nice(current, 19);
- current->flags |= PF_NOFREEZE;
do {
schedule_timeout_uninterruptible(1 + rcu_random(&rand)%10);
@@ -589,7 +587,6 @@ rcu_torture_reader(void *arg)
VERBOSE_PRINTK_STRING("rcu_torture_reader task started");
set_user_nice(current, 19);
- current->flags |= PF_NOFREEZE;
do {
idx = cur_ops->readlock();
Index: linux-2.6.22-rc3/kernel/softirq.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/softirq.c
+++ linux-2.6.22-rc3/kernel/softirq.c
@@ -489,7 +489,6 @@ void __init softirq_init(void)
static int ksoftirqd(void * __bind_cpu)
{
set_user_nice(current, 19);
- current->flags |= PF_NOFREEZE;
set_current_state(TASK_INTERRUPTIBLE);
Index: linux-2.6.22-rc3/kernel/softlockup.c
===================================================================
--- linux-2.6.22-rc3.orig/kernel/softlockup.c
+++ linux-2.6.22-rc3/kernel/softlockup.c
@@ -116,7 +116,6 @@ static int watchdog(void * __bind_cpu)
struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
sched_setscheduler(current, SCHED_FIFO, ¶m);
- current->flags |= PF_NOFREEZE;
/* initialize timestamp */
touch_softlockup_watchdog();
Index: linux-2.6.22-rc3/net/bluetooth/bnep/core.c
===================================================================
--- linux-2.6.22-rc3.orig/net/bluetooth/bnep/core.c
+++ linux-2.6.22-rc3/net/bluetooth/bnep/core.c
@@ -474,7 +474,6 @@ static int bnep_session(void *arg)
daemonize("kbnepd %s", dev->name);
set_user_nice(current, -15);
- current->flags |= PF_NOFREEZE;
init_waitqueue_entry(&wait, current);
add_wait_queue(sk->sk_sleep, &wait);
Index: linux-2.6.22-rc3/net/bluetooth/cmtp/core.c
===================================================================
--- linux-2.6.22-rc3.orig/net/bluetooth/cmtp/core.c
+++ linux-2.6.22-rc3/net/bluetooth/cmtp/core.c
@@ -287,7 +287,6 @@ static int cmtp_session(void *arg)
daemonize("kcmtpd_ctr_%d", session->num);
set_user_nice(current, -15);
- current->flags |= PF_NOFREEZE;
init_waitqueue_entry(&wait, current);
add_wait_queue(sk->sk_sleep, &wait);
Index: linux-2.6.22-rc3/net/bluetooth/hidp/core.c
===================================================================
--- linux-2.6.22-rc3.orig/net/bluetooth/hidp/core.c
+++ linux-2.6.22-rc3/net/bluetooth/hidp/core.c
@@ -547,7 +547,6 @@ static int hidp_session(void *arg)
daemonize("khidpd_%04x%04x", vendor, product);
set_user_nice(current, -15);
- current->flags |= PF_NOFREEZE;
init_waitqueue_entry(&ctrl_wait, current);
init_waitqueue_entry(&intr_wait, current);
Index: linux-2.6.22-rc3/net/bluetooth/rfcomm/core.c
===================================================================
--- linux-2.6.22-rc3.orig/net/bluetooth/rfcomm/core.c
+++ linux-2.6.22-rc3/net/bluetooth/rfcomm/core.c
@@ -1940,7 +1940,6 @@ static int rfcomm_run(void *unused)
daemonize("krfcommd");
set_user_nice(current, -10);
- current->flags |= PF_NOFREEZE;
BT_DBG("");
Index: linux-2.6.22-rc3/fs/lockd/svc.c
===================================================================
--- linux-2.6.22-rc3.orig/fs/lockd/svc.c
+++ linux-2.6.22-rc3/fs/lockd/svc.c
@@ -25,6 +25,7 @@
#include <linux/smp.h>
#include <linux/smp_lock.h>
#include <linux/mutex.h>
+#include <linux/freezer.h>
#include <linux/sunrpc/types.h>
#include <linux/sunrpc/stats.h>
@@ -119,6 +120,7 @@ lockd(struct svc_rqst *rqstp)
complete(&lockd_start_done);
daemonize("lockd");
+ set_freezable();
/* Process request with signals blocked, but allow SIGKILL. */
allow_signal(SIGKILL);
Index: linux-2.6.22-rc3/mm/vmscan.c
===================================================================
--- linux-2.6.22-rc3.orig/mm/vmscan.c
+++ linux-2.6.22-rc3/mm/vmscan.c
@@ -1314,6 +1314,7 @@ static int kswapd(void *p)
* trying to free the first piece of memory in the first place).
*/
tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
+ set_freezable();
order = 0;
for ( ; ; ) {
Index: linux-2.6.22-rc3/Documentation/power/freezing-of-tasks.txt
===================================================================
--- /dev/null
+++ linux-2.6.22-rc3/Documentation/power/freezing-of-tasks.txt
@@ -0,0 +1,160 @@
+Freezing of tasks
+ (C) 2007 Rafael J. Wysocki <rjw@...k.pl>, GPL
+
+I. What is the freezing of tasks?
+
+The freezing of tasks is a mechanism by which user space processes and some
+kernel threads are controlled during hibernation or system-wide suspend (on some
+architectures).
+
+II. How it works?
+
+There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE
+and PF_FREEZER_SKIP (the last one is auxiliary). The tasks that have
+PF_NOFREEZE unset (all user space processes and some kernel threads) are
+regarded as 'freezable' and treated in a special way before the system enters a
+suspend state as well as before a hibernation image is created (in what follows
+we only consider hibernation, but the description also applies to suspend).
+
+Namely, as the first step of the hibernation procedure the function
+freeze_processes() (defined in kernel/power/process.c) is called. It executes
+try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and
+sends a fake signal to each of them. A task that receives such a signal and has
+TIF_FREEZE set, should react to it by calling the refrigerator() function
+(defined in kernel/power/process.c), which sets the task's PF_FROZEN flag,
+changes its state to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is
+cleared for it. Then, we say that the task is 'frozen' and therefore the set of
+functions handling this mechanism is called 'the freezer' (these functions are
+defined in kernel/power/process.c and include/linux/freezer.h). User space
+processes are generally frozen before kernel threads.
+
+It is not recommended to call refrigerator() directly. Instead, it is
+recommended to use the try_to_freeze() function (defined in
+include/linux/freezer.h), that checks the task's TIF_FREEZE flag and makes the
+task enter refrigerator() if the flag is set.
+
+For user space processes try_to_freeze() is called automatically from the
+signal-handling code, but the freezable kernel threads need to call it
+explicitly in suitable places. The code to do this may look like the following:
+
+ do {
+ hub_events();
+ wait_event_interruptible(khubd_wait,
+ !list_empty(&hub_event_list));
+ try_to_freeze();
+ } while (!signal_pending(current));
+
+(from drivers/usb/core/hub.c::hub_thread()).
+
+If a freezable kernel thread fails to call try_to_freeze() after the freezer has
+set TIF_FREEZE for it, the freezing of tasks will fail and the entire
+hibernation operation will be cancelled. For this reason, freezable kernel
+threads must call try_to_freeze() somewhere.
+
+After the system memory state has been restored from a hibernation image and
+devices have been reinitialized, the function thaw_processes() is called in
+order to clear the PF_FROZEN flag for each frozen task. Then, the tasks that
+have been frozen leave refrigerator() and continue running.
+
+III. Which kernel threads are freezable?
+
+Kernel threads are not freezable by default. However, a kernel thread may clear
+PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE
+directly is strongly discouraged). From this point it is regarded as freezable
+and must call try_to_freeze() in a suitable place.
+
+IV. Why do we do that?
+
+Generally speaking, there is a couple of reasons to use the freezing of tasks:
+
+1. The principal reason is to prevent filesystems from being damaged after
+hibernation. Namely, for now we have no simple means of checkpointing
+filesystems, so if there are any modifications made to filesystem data and/or
+metadata on disks, we usually cannot bring them back to the state from before
+the modifications. At the same time each hibernation image contains some
+filesystem-related information that must be consistent with the state of the
+on-disk data and metadata after the system memory state has been restored from
+the image (otherwise the filesystems will be damaged in a nasty way, usually
+making them almost impossible to repair). Therefore we freeze tasks that might
+cause the on-disk filesystems' data and metadata to be modified after the
+hibernation image has been created and before the system is finally powered off.
+The majority of them is user space processes, but if any of kernel threads may
+cause something like this to happen, they have to be freezable.
+
+2. The second reason is to prevent user space processes and some kernel threads
+from interfering with the suspending and resuming of devices. For example, a
+user space process running on a second CPU while we are suspending devices may
+be troublesome and without the freezing of tasks we would need some safeguards
+against race conditions that might occur in such a case.
+
+Although Linus Torvalds doesn't like the freezing of tasks, he said this in one
+of the discussions on LKML (http://lkml.org/lkml/2007/4/27/608):
+
+'> Why we freeze tasks at all or why we freeze kernel threads?
+
+In many ways, "at all".
+
+I _do_ realize the IO request queue issues, and that we cannot actually do
+s2ram with some devices in the middle of a DMA. So we want to be able to
+avoid *that*, there's no question about that. And I suspect that stopping
+user threads and then waiting for a sync is practically one of the easier
+ways to do so.
+
+So in practice, the "at all" may become a "why freeze kernel threads?" and
+freezing user threads I don't find really objectionable.'
+
+Still, there are kernel threads that may want to be freezable. For example, if
+a kernel that belongs to a device driver accesses the device directly, it in
+principle needs to know when the device is suspended, so that it doesn't try to
+access it at that time. However, if the kernel thread is freezable, it will be
+frozen before the driver's .suspend() callback is executed and it will be
+thawed after the driver's .resume() callback has run, so it won't be accessing
+the device while it's suspended.
+
+3. Another reason for freezing tasks is to prevent user space processes from
+realizing that hibernation (or suspend) operation takes place. Ideally, user
+space processes should not notice that such a system-wide operation has occured
+and should continue running without any problems after the restore (or resume
+from suspend). Unfortunately, in the most general case this is quite difficult
+to achieve without the freezing of tasks. Consider, for example, a process
+that depends on the number of CPUs being online while it's running. Since we
+need to disable nonboot CPUs during the hibernation, if this process is not
+frozen, it may notice that the number of CPUs has changed and may start to work
+incorrectly because of that.
+
+V. Are there any problems related to the freezing of tasks?
+
+Yes, there are.
+
+First of all, the freezing of kernel threads may be tricky if they depend one
+on another. For example, if kernel thread A waits for a completion (in the
+TASK_UNINTERRUPTIBLE state) that needs to be done by freezable kernel thread B
+and B is frozen in the meantime, then A will be blocked until B is thawed, which
+may be undesirable. That's why kernel threads are not freezable by default.
+
+Second, there are the following two problems related to the freezing of user
+space processes:
+1. Putting processes into an uninterruptible sleep stuffs up the load average.
+2. Now that we have FUSE, plus the framework for doing device drivers in
+userspace, it gets even more complicated because some userspace processes are
+now doing the sorts of things that kernel threads do
+(https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html).
+
+The problem 1. seems to be fixable, although it hasn't been fixed so far. The
+other one is more serious, but it seems that we can work around it by using
+hibernation (and suspend) notifiers (in that case, though, we won't be able to
+avoid the realization by the user space processes that the hibernation is taking
+place).
+
+There also are problems that the freezing of tasks tends to expose, although
+they are not directly related to it. For example, if request_firmware() is
+called from a device driver's .resume() routine, it will timeout and eventually
+fail, because the user land process that should respond to the request is frozen
+at this point. So, seemingly, the failure is due to the freezing of tasks.
+Suppose, however, that the firmware file is located on a filesystem accessible
+only through the device that needs the firmware. In that case, the system won't
+be able to work normally after the restore regardless of whether or not the
+freezing of tasks is used. Consequently, the problem is not really related to
+the freezing of tasks, since it generally exists regardless. [The solution to
+this particular problem is to keep the firmware in memory after it's loaded for
+the first time and upload if from memory to the device whenever necessary.]
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