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SubjectRe: [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@gmail.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@cn.fujitsu.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.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@chello.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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