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Message-Id: <200806022333.56939.rjw@sisk.pl>
Date: Mon, 2 Jun 2008 23:33:56 +0200
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: Pavel Machek <pavel@....cz>
Cc: Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
akpm@...l.org, pm list <linux-pm@...ts.linux-foundation.org>
Subject: [RFC][PATCH] snapshot: Use pm_mutex for mutual exclusion
On Friday, 23 of May 2008, Rafael J. Wysocki wrote:
> On Friday, 23 of May 2008, Alan Cox wrote:
> > > >
> > > > + lock_kernel();
> > > > +
> > >
> > > Hm, well, I admit I'm a bit ignorant as far as the chardev locking is
> > > concerned, but can you please tell me why would that be wrong if we didn't call
> > > lock_kernel() here at all?
> >
> > I've just been pushing the lock down. If the code already has enough
> > internal locking for things like multiple ioctls in parallel then you can
> > probably kill it entirely - but that needs someone who knows that driver
> > well to decide and evaluate.
>
> Thanks, I'll have a look.
Appended is what I think we can do.
Thanks,
Rafael
---
We can avoid taking the BKL in snapshot_ioctl() if pm_mutex is used to prevent
the ioctls from being executed concurrently.
In addition, although it is only possible to open /dev/snapshot once, the task
which has done that may spawn a child that will inherit the open descriptor,
so in theory they can call snapshot_write(), snapshot_read() and
snapshot_release() concurrently. pm_mutex can also be used for mutual
exclusion in such cases.
Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
---
kernel/power/user.c | 68 ++++++++++++++++++++++++++++++++--------------------
1 file changed, 42 insertions(+), 26 deletions(-)
Index: linux-2.6/kernel/power/user.c
===================================================================
--- linux-2.6.orig/kernel/power/user.c
+++ linux-2.6/kernel/power/user.c
@@ -70,16 +70,22 @@ static int snapshot_open(struct inode *i
struct snapshot_data *data;
int error;
- if (!atomic_add_unless(&snapshot_device_available, -1, 0))
- return -EBUSY;
+ mutex_lock(&pm_mutex);
+
+ if (!atomic_add_unless(&snapshot_device_available, -1, 0)) {
+ error = -EBUSY;
+ goto Unlock;
+ }
if ((filp->f_flags & O_ACCMODE) == O_RDWR) {
atomic_inc(&snapshot_device_available);
- return -ENOSYS;
+ error = -ENOSYS;
+ goto Unlock;
}
if(create_basic_memory_bitmaps()) {
atomic_inc(&snapshot_device_available);
- return -ENOMEM;
+ error = -ENOMEM;
+ goto Unlock;
}
nonseekable_open(inode, filp);
data = &snapshot_state;
@@ -99,33 +105,36 @@ static int snapshot_open(struct inode *i
if (error)
pm_notifier_call_chain(PM_POST_HIBERNATION);
}
- if (error) {
+ if (error)
atomic_inc(&snapshot_device_available);
- return error;
- }
data->frozen = 0;
data->ready = 0;
data->platform_support = 0;
- return 0;
+ Unlock:
+ mutex_unlock(&pm_mutex);
+
+ return error;
}
static int snapshot_release(struct inode *inode, struct file *filp)
{
struct snapshot_data *data;
+ mutex_lock(&pm_mutex);
+
swsusp_free();
free_basic_memory_bitmaps();
data = filp->private_data;
free_all_swap_pages(data->swap);
- if (data->frozen) {
- mutex_lock(&pm_mutex);
+ if (data->frozen)
thaw_processes();
- mutex_unlock(&pm_mutex);
- }
pm_notifier_call_chain(data->mode == O_WRONLY ?
PM_POST_HIBERNATION : PM_POST_RESTORE);
atomic_inc(&snapshot_device_available);
+
+ mutex_unlock(&pm_mutex);
+
return 0;
}
@@ -135,9 +144,13 @@ static ssize_t snapshot_read(struct file
struct snapshot_data *data;
ssize_t res;
+ mutex_lock(&pm_mutex);
+
data = filp->private_data;
- if (!data->ready)
- return -ENODATA;
+ if (!data->ready) {
+ res = -ENODATA;
+ goto Unlock;
+ }
res = snapshot_read_next(&data->handle, count);
if (res > 0) {
if (copy_to_user(buf, data_of(data->handle), res))
@@ -145,6 +158,10 @@ static ssize_t snapshot_read(struct file
else
*offp = data->handle.offset;
}
+
+ Unlock:
+ mutex_unlock(&pm_mutex);
+
return res;
}
@@ -154,6 +171,8 @@ static ssize_t snapshot_write(struct fil
struct snapshot_data *data;
ssize_t res;
+ mutex_lock(&pm_mutex);
+
data = filp->private_data;
res = snapshot_write_next(&data->handle, count);
if (res > 0) {
@@ -162,6 +181,9 @@ static ssize_t snapshot_write(struct fil
else
*offp = data->handle.offset;
}
+
+ mutex_unlock(&pm_mutex);
+
return res;
}
@@ -180,16 +202,16 @@ static long snapshot_ioctl(struct file *
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
- data = filp->private_data;
+ if (!mutex_trylock(&pm_mutex))
+ return -EBUSY;
- lock_kernel();
+ data = filp->private_data;
switch (cmd) {
case SNAPSHOT_FREEZE:
if (data->frozen)
break;
- mutex_lock(&pm_mutex);
printk("Syncing filesystems ... ");
sys_sync();
printk("done.\n");
@@ -197,7 +219,6 @@ static long snapshot_ioctl(struct file *
error = freeze_processes();
if (error)
thaw_processes();
- mutex_unlock(&pm_mutex);
if (!error)
data->frozen = 1;
break;
@@ -205,9 +226,7 @@ static long snapshot_ioctl(struct file *
case SNAPSHOT_UNFREEZE:
if (!data->frozen || data->ready)
break;
- mutex_lock(&pm_mutex);
thaw_processes();
- mutex_unlock(&pm_mutex);
data->frozen = 0;
break;
@@ -310,16 +329,11 @@ static long snapshot_ioctl(struct file *
error = -EPERM;
break;
}
- if (!mutex_trylock(&pm_mutex)) {
- error = -EBUSY;
- break;
- }
/*
* Tasks are frozen and the notifiers have been called with
* PM_HIBERNATION_PREPARE
*/
error = suspend_devices_and_enter(PM_SUSPEND_MEM);
- mutex_unlock(&pm_mutex);
break;
case SNAPSHOT_PLATFORM_SUPPORT:
@@ -392,7 +406,9 @@ static long snapshot_ioctl(struct file *
error = -ENOTTY;
}
- unlock_kernel();
+
+ mutex_unlock(&pm_mutex);
+
return error;
}
--
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