linux-kernel - Re: [PATCH 3/4] kgdb,clocksource: Prevent kernel hang in kernel debugger

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Date:	Tue, 26 Jan 2010 18:01:42 +0800
From:	Dongdong Deng <libfetion@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	mingo@...e.hu, John Stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Magnus Damm <damm@...l.co.jp>
Subject: Re: [PATCH 3/4] kgdb,clocksource: Prevent kernel hang in kernel 
	debugger

On Tue, Jan 26, 2010 at 4:50 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 26 Jan 2010, Martin Schwidefsky wrote:
>> On Mon, 25 Jan 2010 22:26:39 -0600
>> Jason Wessel <jason.wessel@...driver.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@...utronix.de>
>> > CC: Martin Schwidefsky <schwidefsky@...ibm.com>
>> > CC: John Stultz <johnstul@...ibm.com>
>> > CC: Andrew Morton <akpm@...ux-foundation.org>
>> > CC: Magnus Damm <damm@...l.co.jp>
>> > Signed-off-by: Jason Wessel <jason.wessel@...driver.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.


---

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
> --
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View attachment "clocksource_resume_watchdog.patch" of type "text/x-patch" (1239 bytes)