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Message-Id: <200702201929.03776.rjw@sisk.pl>
Date: Tue, 20 Feb 2007 19:29:01 +0100
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: Oleg Nesterov <oleg@...sign.ru>
Cc: ego@...ibm.com, akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu,
vatsa@...ibm.com, dipankar@...ibm.com,
venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
Pavel Machek <pavel@....cz>
Subject: Re: freezer problems
On Tuesday, 20 February 2007 01:32, Rafael J. Wysocki wrote:
> On Tuesday, 20 February 2007 01:12, Oleg Nesterov wrote:
> > On 02/20, Rafael J. Wysocki wrote:
> > >
> > > On Monday, 19 February 2007 23:41, Oleg Nesterov wrote:
> > > > On 02/19, Rafael J. Wysocki wrote:
> > > > >
> > > > > On Monday, 19 February 2007 21:23, Oleg Nesterov wrote:
> > > > >
> > > > > > > @@ -199,6 +189,10 @@ static void thaw_tasks(int thaw_user_spa
> > > > > > >
> > > > > > > do_each_thread(g, p) {
> > > > > > > + if (freezer_should_skip(p))
> > > > > > > + cancel_freezing(p);
> > > > > > > + } while_each_thread(g, p);
> > > > > > > + do_each_thread(g, p) {
> > > > > > > if (!freezeable(p))
> > > > > > > continue;
> > > > > >
> > > > > > Any reason for 2 separate do_each_thread() loops ?
> > > > >
> > > > > Yes. If there is a "freeze" request pending for the vfork parent (TIF_FREEZE
> > > > > set), we have to cancel it before the child is unfrozen, since otherwise the
> > > > > parent may go freezing after we try to reset PF_FROZEN for it.
> > > >
> > > > I see, thanks... thaw_process() doesn't take TIF_FREEZE into account.
> > > >
> > > > But doesn't this mean we have a race?
> > > >
> > > > Suppose that try_to_freeze_tasks() failed. It does cancel_freezing() for each
> > > > process before return, but what if some thread already checked TIF_FREEZE and
> > > > (for simplicity) it is preempted before frozen_process() in refrigerator().
> > > >
> > > > thaw_tasks() runs, ignores this task (P), returns. P gets CPU, and becomes
> > > > frozen, but nobody will thaw it.
> > > >
> > > > No?
> > >
> > > Well, I think this is highly theoretical. Namely, try_to_freeze_tasks() only
> > > fails after the timeout that's currently set to 20 sec., and it yields the CPU
> > > in each iteration of the main loop. The task in question would have to refuse
> > > being frozen for 20 sec. and then suddenly decide to freeze itself right before
> > > try_to_freeze_tasks() checks the timeout for the very last time. Then, it
> > > would have to get preempted at this very moment and stay unfrozen at least
> > > until thaw_tasks() starts running and in fact even longer.
> >
> > Yes, yes, it is pure theroretical,
> >
> > > I think we may avoid this by making try_to_freeze_tasks() sleep for some time
> > > after it has reset TIF_FREEZE for all tasks in the error path, if anyone is
> > > ever able to trigger it.
> >
> > This makes this race (pure theroretical) ** 2 :)
> >
> > Still. May be it make sense to introduce cancel_freezing_and_thaw() function
> > (not right now) which stops the task from sleeping in refrigirator reliably.
>
> Hm. In the case discussed above we have a task that's right before calling
> frozen_process(), so we can't thaw it, because it's not frozen. It will be
> frozen just in a while, but try_to_freeze_tasks() and thaw_tasks() have no
> way to check this.
>
> I think to close this race the refrigerator should check TIF_FREEZE and set
> PF_FROZEN _and_ reset TIF_FREEZE under a lock that would also have to be
> taken by try_to_freeze_tasks() in the beginning of the error path. This will
> ensure that all tasks either freeze themselves before the error path in
> try_to_freeze_tasks() is executed, or remain unfrozen.
>
> I'll try to prepare a patch to illustrate this, but right now I'm too tired to
> do it. :-)
Something like this, perhaps:
---
include/linux/freezer.h | 10 +++-------
kernel/power/process.c | 18 ++++++++++++++++--
2 files changed, 19 insertions(+), 9 deletions(-)
Index: linux-2.6.20-mm2/include/linux/freezer.h
===================================================================
--- linux-2.6.20-mm2.orig/include/linux/freezer.h
+++ linux-2.6.20-mm2/include/linux/freezer.h
@@ -58,17 +58,13 @@ static inline void frozen_process(struct
clear_tsk_thread_flag(p, TIF_FREEZE);
}
-extern void refrigerator(void);
+extern int refrigerator(void);
extern int freeze_processes(void);
extern void thaw_processes(void);
static inline int try_to_freeze(void)
{
- if (freezing(current)) {
- refrigerator();
- return 1;
- } else
- return 0;
+ return refrigerator();
}
/*
@@ -104,7 +100,7 @@ static inline void freeze(struct task_st
static inline int thaw_process(struct task_struct *p) { return 1; }
static inline void frozen_process(struct task_struct *p) { BUG(); }
-static inline void refrigerator(void) {}
+static inline int refrigerator(void) { return 0; }
static inline int freeze_processes(void) { BUG(); return 0; }
static inline void thaw_processes(void) {}
Index: linux-2.6.20-mm2/kernel/power/process.c
===================================================================
--- linux-2.6.20-mm2.orig/kernel/power/process.c
+++ linux-2.6.20-mm2/kernel/power/process.c
@@ -24,6 +24,8 @@
#define FREEZER_KERNEL_THREADS 0
#define FREEZER_USER_SPACE 1
+spinlock_t refrigerator_lock;
+
static inline int freezeable(struct task_struct * p)
{
if ((p == current) ||
@@ -34,15 +36,23 @@ static inline int freezeable(struct task
}
/* Refrigerator is place where frozen processes are stored :-). */
-void refrigerator(void)
+int refrigerator(void)
{
/* Hmm, should we be allowed to suspend when there are realtime
processes around? */
long save;
+
+ spin_lock(&refrigerator_lock);
+ if (freezing(current)) {
+ frozen_process(current);
+ spin_unlock(&refrigerator_lock);
+ } else {
+ spin_unlock(&refrigerator_lock);
+ return 0;
+ }
save = current->state;
pr_debug("%s entered refrigerator\n", current->comm);
- frozen_process(current);
spin_lock_irq(¤t->sighand->siglock);
recalc_sigpending(); /* We sent fake signal, clean it up */
spin_unlock_irq(¤t->sighand->siglock);
@@ -53,6 +63,7 @@ void refrigerator(void)
}
pr_debug("%s left refrigerator\n", current->comm);
current->state = save;
+ return 1;
}
static inline void freeze_process(struct task_struct *p)
@@ -143,6 +154,7 @@ static unsigned int try_to_freeze_tasks(
"kernel threads",
TIMEOUT / HZ, todo);
read_lock(&tasklist_lock);
+ spin_lock(&refrigerator_lock);
do_each_thread(g, p) {
if (is_user_space(p) == !freeze_user_space)
continue;
@@ -152,6 +164,7 @@ static unsigned int try_to_freeze_tasks(
cancel_freezing(p);
} while_each_thread(g, p);
+ spin_unlock(&refrigerator_lock);
read_unlock(&tasklist_lock);
}
@@ -169,6 +182,7 @@ int freeze_processes(void)
unsigned int nr_unfrozen;
printk("Stopping tasks ... ");
+ spin_lock_init(&refrigerator_lock);
nr_unfrozen = try_to_freeze_tasks(FREEZER_USER_SPACE);
if (nr_unfrozen)
return nr_unfrozen;
-
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