On Fri, Feb 20, 2015 at 09:23:19PM +0100, Oleg Nesterov wrote: > 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?
Sure, that works. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
