linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Message-ID: <20151103182304.GF17308@twins.programming.kicks-ass.net>
Date:	Tue, 3 Nov 2015 19:23:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, boqun.feng@...il.com, corbet@....net,
	mhocko@...nel.org, dhowells@...hat.com,
	torvalds@...ux-foundation.org, will.deacon@....com
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Tue, Nov 03, 2015 at 06:59:58PM +0100, Oleg Nesterov wrote:

> > -		raw_spin_unlock_wait(&task->pi_lock);
> > -		smp_mb();
> > +		smp_cond_acquire(!raw_spin_is_locked(&task->pi_lock));
> 
> Unfortunately this doesn't look exactly right...
> 
> spin_unlock_wait() is not equal to "while (locked) relax", the latter
> is live-lockable or at least sub-optimal: we do not really need to spin
> until we observe !spin_is_locked(), we only need to synchronize with the
> current owner of this lock. Once it drops the lock we can proceed, we
> do not care if another thread takes the same lock right after that.

Ah indeed. And while every use of spin_unlock_wait() has 'interesting'
barriers associated, they all seem different.
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