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 > > 15:23:48.000000000 +0200 > > +++ linux-2.6.21-rc6-mm1/kernel/kthread.c 2007-04-22 19:05:29.000000000 > > +0200 > > @@ -13,6 +13,7 @@ > > #include <linux/file.h> > > #include <linux/module.h> > > #include <linux/mutex.h> > > +#include <linux/freezer.h> > > #include <asm/semaphore.h> > > > > /* > > @@ -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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/