linux-kernel - Re: [PATCH 2/2] [PATCH] sched: Add smp

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Date:	Sat, 21 Feb 2015 13:54:38 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles

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.
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