Re: wake_up_process implied memory barrier clarification

From: Peter Zijlstra
Date: Thu Aug 27 2015 - 08:43:44 EST

On Thu, Aug 27, 2015 at 02:27:27PM +0200, Michal Hocko wrote:
> Hi,
> I have just stumbled over the comment above wake_up_process which
> claims:
> "
> * It may be assumed that this function implies a write memory barrier before
> * changing the task state if and only if any tasks are woken up.
> "
> but try_to_wake_up does smp_mb__before_spinlock and did smp_wmb
> since 04e2f1741d235 unconditionally. The comment was added when the
> smp_wmb was in place already so I am wondering whether the comment is
> wrong/misleading.
> Could somebody clarify please?

Its true for wake_up(), since that bails early if the waitqueue list is

I suspect there was no exception made for wake_up_process() to simplify
the rules.
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