Messages in this thread | | | Date | Tue, 3 Nov 2015 18:59:58 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() |
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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.
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