Messages in this thread | | | Date | Fri, 20 Feb 2015 21:23:19 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles |
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On 02/20, Peter Zijlstra wrote: > > I think I agree with Oleg in that we only need the smp_rmb(); of course > that wants a somewhat elaborate comment to go along with it. How about > something like so: > > spin_unlock_wait(&local); > /* > * The above spin_unlock_wait() forms a control dependency with > * any following stores; because we must first observe the lock > * unlocked and we cannot speculate stores. > * > * Subsequent loads however can easily pass through the loads > * represented by spin_unlock_wait() and therefore we need the > * read barrier. > * > * This together is stronger than ACQUIRE for @local and > * therefore we will observe the complete prior critical section > * of @local. > */ > smp_rmb(); > > The obvious alternative is using spin_unlock_wait() with an > smp_load_acquire(), but that might be more expensive on some archs due > to repeated issuing of memory barriers.
Yes, yes, thanks!
But note that we need the same comment after sem_lock()->spin_is_locked().
So perhaps we can add this comment into include/linux/spinlock.h ? In this case perhaps it makes sense to add, say,
#define smp_mb__after_unlock_wait() smp_rmb()
with this comment above? Another potential user task_work_run(). It could use rmb() too, but this again needs the same fat comment.
Ehat do you think?
Oleg.
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