Messages in this thread | | | Date | Fri, 20 Feb 2015 19:45:51 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles |
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On Fri, Feb 20, 2015 at 07:28:16PM +0100, Manfred Spraul wrote:
> >We need the full barrier to serialize STORE's as well, but probably we can > >rely on control dependancy and thus we only need rmb(). > Do we need a full barrier or not? > > I don't manage to create a proper line of reasoning.
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.
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