linux-kernel - Re: [tip:sched/core] sched: Pull up the might_sleep() check into cond

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Date:	Mon, 20 Jul 2009 13:56:38 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, hpa@...or.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Pull up the might_sleep() check into
	cond_resched()


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Mon, 2009-07-20 at 04:12 -0400, Frederic Weisbecker wrote:
> 
> > From: Frederic Weisbecker <fweisbec@...il.com>
> > Subject: [PATCH] sched: Check if the spinlock is locked in cond_resched_lock()
> > 
> > Some uses of cond_resched_lock() might involve an
> > unlocked spinlock, resulting in spurious sleep in
> > atomic warnings.
> > Check whether the spinlock is actually locked and
> > take that into account in the might_sleep() check.
> > 
> > Reported-by: Li Zefan <lizf@...fujitsu.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > ---
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index cb070dc..2789658 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -2294,9 +2294,10 @@ extern int _cond_resched(void);
> >  
> >  extern int __cond_resched_lock(spinlock_t *lock);
> >  
> > -#define cond_resched_lock(lock) ({				\
> > -	__might_sleep(__FILE__, __LINE__, PREEMPT_OFFSET);	\
> > -	__cond_resched_lock(lock);				\
> > +#define cond_resched_lock(lock) ({				  \
> > +	__might_sleep(__FILE__, __LINE__, spin_is_locked(lock) ?  \
> > +					  PREEMPT_OFFSET : 0);	  \
> > +	__cond_resched_lock(lock);				  \
> >  })
> >  
> >  extern int __cond_resched_softirq(void);
> 
> 
> No, this looks utterly broken.. who is to say it doesn't get unlocked
> right after that spin_is_locked() check?
> 
> cond_resched_lock() callers must hold the lock they use it on, not doing
> so is broken.
> 
> So I would suggest something like the below instead:
> 
> (utterly untested)

FYI, i've undone the tip:sched/core bits from tip:master for now - 
please send a delta patch against tip:sched/core once this is fixed.

Thanks,

	Ingo
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