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

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Date:	Mon, 20 Jul 2009 05:13:11 -0400
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Li Zefan <lizf@...fujitsu.com>, hpa@...or.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Pull up the might_sleep() check into
	cond_resched()

On Mon, Jul 20, 2009 at 10:49:07AM +0200, Peter Zijlstra 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?


Oh, indeed, that was silly...


> 
> 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)


Looks better anyway.

Thanks,
Frederic.


> 
> Almost-signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  include/linux/lockdep.h |    8 ++++++++
>  kernel/lockdep.c        |   33 +++++++++++++++++++++++++++++++++
>  kernel/sched.c          |    2 ++
>  3 files changed, 43 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 12aabfc..920df42 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -296,6 +296,10 @@ extern void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>  extern void lock_release(struct lockdep_map *lock, int nested,
>  			 unsigned long ip);
>  
> +#define lockdep_is_held(lock)	lock_is_held(&(lock)->dep_map)
> +
> +extern int lock_is_held(struct lockdep_map *lock);
> +
>  extern void lock_set_class(struct lockdep_map *lock, const char *name,
>  			   struct lock_class_key *key, unsigned int subclass,
>  			   unsigned long ip);
> @@ -314,6 +318,8 @@ extern void lockdep_trace_alloc(gfp_t mask);
>  
>  #define lockdep_depth(tsk)	(debug_locks ? (tsk)->lockdep_depth : 0)
>  
> +#define lockdep_assert_held(l)	WARN_ON(!lockdep_is_held(l))
> +
>  #else /* !LOCKDEP */
>  
>  static inline void lockdep_off(void)
> @@ -358,6 +364,8 @@ struct lock_class_key { };
>  
>  #define lockdep_depth(tsk)	(0)
>  
> +#define lockdep_assert_held(l)			do { } while (0)
> +
>  #endif /* !LOCKDEP */
>  
>  #ifdef CONFIG_LOCK_STAT
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 3718a98..fd626ea 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -3044,6 +3044,19 @@ __lock_release(struct lockdep_map *lock, int nested, unsigned long ip)
>  	check_chain_key(curr);
>  }
>  
> +static int __lock_is_held(struct lockdep_map *lock)
> +{
> +	struct task_struct *curr = current;
> +	int i;
> +
> +	for (i = 0; i < curr->lockdep_depth; i++) {
> +		if (curr->held_locks[i].instance == lock)
> +			return 1;
> +	}
> +
> +	return 0;
> +}
> +
>  /*
>   * Check whether we follow the irq-flags state precisely:
>   */
> @@ -3145,6 +3158,26 @@ void lock_release(struct lockdep_map *lock, int nested,
>  }
>  EXPORT_SYMBOL_GPL(lock_release);
>  
> +int lock_is_held(struct lockdep_map *lock)
> +{
> +	unsigned long flags;
> +	int ret = 0;
> +
> +	if (unlikely(current->lockdep_recursion))
> +		return;
> +
> +	raw_local_irq_save(flags);
> +	check_flags(flags);
> +
> +	current->lockdep_recursion = 1;
> +	ret = __lock_is_held(lock);
> +	current->lockdep_recursion = 0;
> +	raw_local_irq_restore(flags);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(lock_is_held);
> +
>  void lockdep_set_current_reclaim_state(gfp_t gfp_mask)
>  {
>  	current->lockdep_reclaim_gfp = gfp_mask;
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 56fb225..6255f82 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -6638,6 +6638,8 @@ int __cond_resched_lock(spinlock_t *lock)
>  	int resched = should_resched();
>  	int ret = 0;
>  
> +	lockdep_assert_held(lock);
> +
>  	if (spin_needbreak(lock) || resched) {
>  		spin_unlock(lock);
>  		if (resched)
> 
> 

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