Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

Oleg Nesterov
2015-11-03T17:03:47Z

On 11/02, Peter Zijlstra wrote:
>
> +#define smp_cond_acquire(cond) do { \
> + while (!(cond)) \
> + cpu_relax(); \
> + smp_read_barrier_depends(); /* ctrl */ \
> + smp_rmb(); /* ctrl + rmb := acquire */ \
> +} while (0)

...

> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -102,13 +102,13 @@ void task_work_run(void)
>
> if (!work)
> break;
> +
> /*
> * Synchronize with task_work_cancel(). It can't remove
> * the first entry == work, cmpxchg(task_works) should
> * fail, but it can play with *work and other entries.
> */
> - raw_spin_unlock_wait(&task->pi_lock);
> - smp_mb();
> + smp_cond_acquire(!raw_spin_is_locked(&task->pi_lock));

Unfortunately this doesn't look exactly right...

spin_unlock_wait() is not equal to "while (locked) relax", the latter
is live-lockable or at least sub-optimal: we do not really need to spin
until we observe !spin_is_locked(), we only need to synchronize with the
current owner of this lock. Once it drops the lock we can proceed, we
do not care if another thread takes the same lock right after that.

Oleg.