Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
On Monday, 23 April 2007 23:16, Gautham R Shenoy wrote:
> On Tue, Apr 24, 2007 at 12:46:37AM +0400, Oleg Nesterov wrote:
> > On 04/23, Rafael J. Wysocki wrote:
> > >
> > > On Monday, 23 April 2007 14:35, Gautham R Shenoy wrote:
> > > > > + if (!freezer_should_exempt(current)) {
> > > > task_lock(k);
> > > > > + /* We are freezable, so we must make sure that the
> > > > > thread being
> > > > > + * stopped is not frozen and will not be frozen until
> > > > > it dies
> > > > > + */
> > > > > + freezer_exempt(k);
> > > > > + if (frozen(k))
> > > > > + clear_frozen_flag(k);
> > > > task_unlock(k);
> > > > > + }
> > >
> > > Yes, that's correct. We need to take task_lock() to avoid the race with
> > > refrigerator().
> >
> > Even if we use thaw_task() ?
>
> I don't think so. As you correctly pointed out, thaw_task() is race free
> w.r.t the refrigerator().
Agreed.
Rafael
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
On Tue, Apr 24, 2007 at 12:46:37AM +0400, Oleg Nesterov wrote:
> On 04/23, Rafael J. Wysocki wrote:
> >
> > On Monday, 23 April 2007 14:35, Gautham R Shenoy wrote:
> > > > + if (!freezer_should_exempt(current)) {
> > > task_lock(k);
> > > > + /* We are freezable, so we must make sure that the
> > > > thread being
> > > > +* stopped is not frozen and will not be frozen until
> > > > it dies
> > > > +*/
> > > > + freezer_exempt(k);
> > > > + if (frozen(k))
> > > > + clear_frozen_flag(k);
> > > task_unlock(k);
> > > > + }
> >
> > Yes, that's correct. We need to take task_lock() to avoid the race with
> > refrigerator().
>
> Even if we use thaw_task() ?
I don't think so. As you correctly pointed out, thaw_task() is race free
w.r.t the refrigerator().
>
> Even if I am wrong, I think we should not use task_lock() for the freezing
> related code, except in freezer.[ch]
>
> Note also that without CONFIG_FREEZER freezer_should_exempt() == 0, so we
> will do unneeded task_lock/task_unlock.
>
> Oleg.
>
Thanks and Regards
gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
On 04/23, Rafael J. Wysocki wrote:
>
> On Monday, 23 April 2007 14:35, Gautham R Shenoy wrote:
> > > + if (!freezer_should_exempt(current)) {
> > task_lock(k);
> > > + /* We are freezable, so we must make sure that the thread being
> > > + * stopped is not frozen and will not be frozen until it dies
> > > + */
> > > + freezer_exempt(k);
> > > + if (frozen(k))
> > > + clear_frozen_flag(k);
> > task_unlock(k);
> > > + }
>
> Yes, that's correct. We need to take task_lock() to avoid the race with
> refrigerator().
Even if we use thaw_task() ?
Even if I am wrong, I think we should not use task_lock() for the freezing
related code, except in freezer.[ch]
Note also that without CONFIG_FREEZER freezer_should_exempt() == 0, so we
will do unneeded task_lock/task_unlock.
Oleg.
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
On Monday, 23 April 2007 21:03, Oleg Nesterov wrote:
> On 04/23, Gautham R Shenoy wrote:
> >
> > On Sun, Apr 22, 2007 at 09:40:59PM +0200, Rafael J. Wysocki wrote:
> > > /*
> > > @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
> > >
> > > /* Now set kthread_should_stop() to true, and wake it up. */
> > > kthread_stop_info.k = k;
> > > + if (!freezer_should_exempt(current)) {
> > > + /* We are freezable, so we must make sure that the thread being
> > > + * stopped is not frozen and will not be frozen until it dies
> > > + */
> > > + freezer_exempt(k);
> > > + if (frozen(k))
> > > + clear_frozen_flag(k);
> > > + }
> >
> > I'm trying hard to convince myself that this will work. May be I am
> > missing something here, but I find a potential race window (very small
> > though)
> > when k is entering the refrigerator.
> >
> > [... snip ... ]
> >
> > IMO, we need the to take the task_lock for k here. Something like
> >
> > > + if (!freezer_should_exempt(current)) {
> > task_lock(k);
> > > + /* We are freezable, so we must make sure that the thread being
> > > + * stopped is not frozen and will not be frozen until it dies
> > > + */
> > > + freezer_exempt(k);
> > > + if (frozen(k))
> > > + clear_frozen_flag(k);
> > task_unlock(k);
> > > + }
>
> Well, probably I missed something, but why can't we do
>
> if (!freezer_should_exempt(current)) {
> freezer_exempt(k);
> thaw_process(k);
> }
> ?
>
> thaw_process(k) is properly serialized with refrigerator(), and it checks
> frozen(k). We can make an extra wake_up, but this should not matter.
Yes, I think you're right.
> Rafael, please check the recent changes in kthread.c, kthread_stop() was
> reworked, we don't have kthread_stop_info any longer.
OK, I will, thanks.
Greetings,
Rafael
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
Hi,
On Monday, 23 April 2007 12:40, Pavel Machek wrote:
> Hi!
>
> > Fix the problem with kthread_stop() that causes the freezer to fail if a
> > freezable thread is attempting to stop a frozen one and that may cause the
> > freezer to fail if the thread being stopped is freezable and
> > try_to_freeze_tasks() is running concurrently with kthread_stop().
>
> Parse error.
OK, will fix.
> > Index: linux-2.6.21-rc6-mm1/kernel/kthread.c
> > ===
> > --- linux-2.6.21-rc6-mm1.orig/kernel/kthread.c 2007-04-09
> > 15:23:48.0 +0200
> > +++ linux-2.6.21-rc6-mm1/kernel/kthread.c 2007-04-22 19:05:29.0
> > +0200
> > @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
> >
> > /* Now set kthread_should_stop() to true, and wake it up. */
> > kthread_stop_info.k = k;
> > + if (!freezer_should_exempt(current)) {
> > + /* We are freezable, so we must make sure that the thread being
> > +* stopped is not frozen and will not be frozen until it dies
> > +*/
> > + freezer_exempt(k);
> > + if (frozen(k))
> > + clear_frozen_flag(k);
> > + }
> > wake_up_process(k);
> > put_task_struct(k);
> >
>
> Do we need to take some locks to access other process' flags? Or do
> frozen_exempt() etc take enough locks, already?
After the previous patch we only use set_bit(), clear_bit() and test_bit() to
access freezer_falgs, so no special locking is needed to protect them.
Greetings,
Rafael
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
On Monday, 23 April 2007 14:35, Gautham R Shenoy wrote:
> On Sun, Apr 22, 2007 at 09:40:59PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <[EMAIL PROTECTED]>
> >
> > Fix the problem with kthread_stop() that causes the freezer to fail if a
> > freezable thread is attempting to stop a frozen one and that may cause the
> > freezer to fail if the thread being stopped is freezable and
> > try_to_freeze_tasks() is running concurrently with kthread_stop().
> >
> > Signed-off-by: Rafael J. Wysocki <[EMAIL PROTECTED]>
> > ---
> > kernel/kthread.c |9 +
> > 1 file changed, 9 insertions(+)
> >
> > Index: linux-2.6.21-rc6-mm1/kernel/kthread.c
> > ===
> > --- linux-2.6.21-rc6-mm1.orig/kernel/kthread.c 2007-04-09
> > 15:23:48.0 +0200
> > +++ linux-2.6.21-rc6-mm1/kernel/kthread.c 2007-04-22 19:05:29.0
> > +0200
> > @@ -13,6 +13,7 @@
> > #include
> > #include
> > #include
> > +#include
> > #include
> >
> > /*
> > @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
> >
> > /* Now set kthread_should_stop() to true, and wake it up. */
> > kthread_stop_info.k = k;
> > + if (!freezer_should_exempt(current)) {
> > + /* We are freezable, so we must make sure that the thread being
> > +* stopped is not frozen and will not be frozen until it dies
> > +*/
> > + freezer_exempt(k);
> > + if (frozen(k))
> > + clear_frozen_flag(k);
> > + }
>
> I'm trying hard to convince myself that this will work. May be I am
> missing something here, but I find a potential race window (very small
> though)
> when k is entering the refrigerator.
>
> Here's how.
>
> kthread_stop(k) k->refrigerator()
> -
> task_lock(k);
> 1) check_if_exempted();
> /* not exempted. So
>* we will freeze
>* ourself.
>*/
> 2) freezer_exempt(k);
>
> 3) if(frozen(k))
> /* No, we're not yet frozen. So we
> * don't clear_frozen_flag(k) here
> */
> 4) frozen_process(k);
> task_unlock(k);
>
> 5) for(;;) {
>
> set_current_state(UNINTERRUPTIBLE);
> if(!frozen_process(k))
> /* k is frozen. We
> * don't break :(
> */
>
> schedule();
> }
>
> > wake_up_process(k);
> > put_task_struct(k);
> >
>
> Thus the freezer can still fail, no?
> IMO, we need the to take the task_lock for k here. Something like
>
> > + if (!freezer_should_exempt(current)) {
> task_lock(k);
> > + /* We are freezable, so we must make sure that the thread being
> > +* stopped is not frozen and will not be frozen until it dies
> > +*/
> > + freezer_exempt(k);
> > + if (frozen(k))
> > + clear_frozen_flag(k);
> task_unlock(k);
> > + }
Yes, that's correct. We need to take task_lock() to avoid the race with
refrigerator().
I'll fix the patch.
BTW, I think I should rediff the entire series against -mm with your patch from
http://lkml.org/lkml/2007/4/23/98 applied.
Greetings,
Rafael
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
On 04/23, Gautham R Shenoy wrote:
>
> On Sun, Apr 22, 2007 at 09:40:59PM +0200, Rafael J. Wysocki wrote:
> > /*
> > @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
> >
> > /* Now set kthread_should_stop() to true, and wake it up. */
> > kthread_stop_info.k = k;
> > + if (!freezer_should_exempt(current)) {
> > + /* We are freezable, so we must make sure that the thread being
> > +* stopped is not frozen and will not be frozen until it dies
> > +*/
> > + freezer_exempt(k);
> > + if (frozen(k))
> > + clear_frozen_flag(k);
> > + }
>
> I'm trying hard to convince myself that this will work. May be I am
> missing something here, but I find a potential race window (very small
> though)
> when k is entering the refrigerator.
>
> [... snip ... ]
>
> IMO, we need the to take the task_lock for k here. Something like
>
> > + if (!freezer_should_exempt(current)) {
> task_lock(k);
> > + /* We are freezable, so we must make sure that the thread being
> > +* stopped is not frozen and will not be frozen until it dies
> > +*/
> > + freezer_exempt(k);
> > + if (frozen(k))
> > + clear_frozen_flag(k);
> task_unlock(k);
> > + }
Well, probably I missed something, but why can't we do
if (!freezer_should_exempt(current)) {
freezer_exempt(k);
thaw_process(k);
}
?
thaw_process(k) is properly serialized with refrigerator(), and it checks
frozen(k). We can make an extra wake_up, but this should not matter.
Rafael, please check the recent changes in kthread.c, kthread_stop() was
reworked, we don't have kthread_stop_info any longer.
Oleg.
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
On Sun, Apr 22, 2007 at 09:40:59PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <[EMAIL PROTECTED]>
>
> Fix the problem with kthread_stop() that causes the freezer to fail if a
> freezable thread is attempting to stop a frozen one and that may cause the
> freezer to fail if the thread being stopped is freezable and
> try_to_freeze_tasks() is running concurrently with kthread_stop().
>
> Signed-off-by: Rafael J. Wysocki <[EMAIL PROTECTED]>
> ---
> kernel/kthread.c |9 +
> 1 file changed, 9 insertions(+)
>
> Index: linux-2.6.21-rc6-mm1/kernel/kthread.c
> ===
> --- linux-2.6.21-rc6-mm1.orig/kernel/kthread.c2007-04-09
> 15:23:48.0 +0200
> +++ linux-2.6.21-rc6-mm1/kernel/kthread.c 2007-04-22 19:05:29.0
> +0200
> @@ -13,6 +13,7 @@
> #include
> #include
> #include
> +#include
> #include
>
> /*
> @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
>
> /* Now set kthread_should_stop() to true, and wake it up. */
> kthread_stop_info.k = k;
> + if (!freezer_should_exempt(current)) {
> + /* We are freezable, so we must make sure that the thread being
> + * stopped is not frozen and will not be frozen until it dies
> + */
> + freezer_exempt(k);
> + if (frozen(k))
> + clear_frozen_flag(k);
> + }
I'm trying hard to convince myself that this will work. May be I am
missing something here, but I find a potential race window (very small though)
when k is entering the refrigerator.
Here's how.
kthread_stop(k) k->refrigerator()
-
task_lock(k);
1) check_if_exempted();
/* not exempted. So
* we will freeze
* ourself.
*/
2) freezer_exempt(k);
3) if(frozen(k))
/* No, we're not yet frozen. So we
* don't clear_frozen_flag(k) here
*/
4) frozen_process(k);
task_unlock(k);
5) for(;;) {
set_current_state(UNINTERRUPTIBLE);
if(!frozen_process(k))
/* k is frozen. We
* don't break :(
*/
schedule();
}
> wake_up_process(k);
> put_task_struct(k);
>
Thus the freezer can still fail, no?
IMO, we need the to take the task_lock for k here. Something like
> + if (!freezer_should_exempt(current)) {
task_lock(k);
> + /* We are freezable, so we must make sure that the thread being
> + * stopped is not frozen and will not be frozen until it dies
> + */
> + freezer_exempt(k);
> + if (frozen(k))
> + clear_frozen_flag(k);
task_unlock(k);
> + }
Thanks and Regards
gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop
Hi!
> Fix the problem with kthread_stop() that causes the freezer to fail if a
> freezable thread is attempting to stop a frozen one and that may cause the
> freezer to fail if the thread being stopped is freezable and
> try_to_freeze_tasks() is running concurrently with kthread_stop().
Parse error.
> Index: linux-2.6.21-rc6-mm1/kernel/kthread.c
> ===
> --- linux-2.6.21-rc6-mm1.orig/kernel/kthread.c2007-04-09
> 15:23:48.0 +0200
> +++ linux-2.6.21-rc6-mm1/kernel/kthread.c 2007-04-22 19:05:29.0
> +0200
> @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
>
> /* Now set kthread_should_stop() to true, and wake it up. */
> kthread_stop_info.k = k;
> + if (!freezer_should_exempt(current)) {
> + /* We are freezable, so we must make sure that the thread being
> + * stopped is not frozen and will not be frozen until it dies
> + */
> + freezer_exempt(k);
> + if (frozen(k))
> + clear_frozen_flag(k);
> + }
> wake_up_process(k);
> put_task_struct(k);
>
Do we need to take some locks to access other process' flags? Or do
frozen_exempt() etc take enough locks, already?
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures)
http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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