Messages in this thread | | | Date | Tue, 26 Jan 2010 18:19:32 +0800 | Subject | Re: [PATCH 3/4] kgdb,clocksource: Prevent kernel hang in kernel debugger | From | Xiaotian Feng <> |
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On Tue, Jan 26, 2010 at 6:01 PM, Dongdong Deng <libfetion@gmail.com> wrote: > On Tue, Jan 26, 2010 at 4:50 PM, Thomas Gleixner <tglx@linutronix.de> wrote: >> On Tue, 26 Jan 2010, Martin Schwidefsky wrote: >>> On Mon, 25 Jan 2010 22:26:39 -0600 >>> Jason Wessel <jason.wessel@windriver.com> wrote: >>> >>> > This is a regression fix against: 0f8e8ef7c204988246da5a42d576b7fa5277a8e4 >>> > >>> > Spin locks were added to the clocksource_resume_watchdog() which cause >>> > the kernel debugger to deadlock on an SMP system frequently. >>> > >>> > The kernel debugger can try for the lock, but if it fails it should >>> > continue to touch the clocksource watchdog anyway, else it will trip >>> > if the general kernel execution has been paused for too long. >>> > >>> > This introduces an possible race condition where the kernel debugger >>> > might not process the list correctly if a clocksource is being added >>> > or removed at the time of this call. This race is sufficiently rare vs >>> > having the kernel debugger hang the kernel >>> > >>> > CC: Thomas Gleixner <tglx@linutronix.de> >>> > CC: Martin Schwidefsky <schwidefsky@de.ibm.com> >>> > CC: John Stultz <johnstul@us.ibm.com> >>> > CC: Andrew Morton <akpm@linux-foundation.org> >>> > CC: Magnus Damm <damm@igel.co.jp> >>> > Signed-off-by: Jason Wessel <jason.wessel@windriver.com> >>> >>> The first question I would ask is why does the kernel deadlock? Can we >>> have a backchain of a deadlock please? >> >> The problem arises when the kernel is stopped inside the watchdog code >> with watchdog_lock held. When kgdb restarts execution then it touches >> the watchdog to avoid that TSC gets marked unstable. >> >>> Hmm, there are all kinds of races if the watchdog code gets interrupted >>> by the kernel debugger. Wouldn't it be better to just disable the >>> watchdog while the kernel debugger is active? >> >> No, we can keep it and in most cases it clocksource_touch_watchdog() >> helps to keep TSC alive. A simple "if (!trylock) return;" should solve >> the deadlock problem for kgdb without opening a can of worms. > > Is it possible that we reset the clocksource watchdog during in > clocksource_watchdog() ? > > From the code view, The action of reset clocksource watchdog is just > set the CLOCK_SOURCE_WATCHDOG flag. > thus if we reset it before using, I think the logic will be right.
Is this logic right for resume case?
> > > --- > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c > index e85c234..0590983 100644 > --- a/kernel/time/clocksource.c > +++ b/kernel/time/clocksource.c > @@ -187,6 +187,8 @@ static DEFINE_SPINLOCK(watchdog_lock); > static cycle_t watchdog_last; > static int watchdog_running; > > +static int volatile clocksource_resume_watchdog; > + > static int clocksource_watchdog_kthread(void *data); > static void __clocksource_change_rating(struct clocksource *cs, int rating); > > @@ -253,6 +255,11 @@ static void clocksource_watchdog(unsigned long data) > if (!watchdog_running) > goto out; > > + if (unlikely(clocksource_resume_watchdog)) { > + clocksource_reset_watchdog(); > + clocksource_resume_watchdog = 0; > + } > + > wdnow = watchdog->read(watchdog); > wd_nsec = clocksource_cyc2ns((wdnow - watchdog_last) & watchdog->mask, > watchdog->mult, watchdog->shift); > @@ -341,11 +348,7 @@ static inline void clocksource_reset_watchdog(void) > > static void clocksource_resume_watchdog(void) > { > - unsigned long flags; > - > - spin_lock_irqsave(&watchdog_lock, flags); > - clocksource_reset_watchdog(); > - spin_unlock_irqrestore(&watchdog_lock, flags); > + clocksource_resume_watchdog = 1; > } > > static void clocksource_enqueue_watchdog(struct clocksource *cs) > > > >> >> Thanks, >> >> tglx >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ >> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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