linux-kernel - Re: [RFC][PATCH v2 4/5] mutex: Disable preemtion between modifying lock->owner and locking/unlocking mutex

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Date:	Tue, 28 Jan 2014 14:17:49 -0800
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, paulmck@...ux.vnet.ibm.com, Waiman.Long@...com,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	andi@...stfloor.org, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [RFC][PATCH v2 4/5] mutex: Disable preemtion between modifying
 lock->owner and locking/unlocking mutex

On Tue, 2014-01-28 at 21:54 +0100, Peter Zijlstra wrote:
> On Tue, Jan 28, 2014 at 11:13:15AM -0800, Jason Low wrote:
> > This RFC patch disables preemption between modifying lock->owner and
> > locking/unlocking the mutex lock. This prevents situations where the owner
> > can preempt between those 2 operations, which causes optimistic spinners to
> > be unable to check if lock->owner is not on CPU. As mentioned in the
> > thread for this v1 patchset, disabling preemption is a cheap operation.
> 
> In that same thread it was also said that this wasn't really an issue at
> all. So what is the justification?

This patch is mainly just a complementary patch for patch 5 so that we
can allow the spinner to wait slightly longer for lock->count to get set
without worrying about need_resched()/owner getting preempted.

Jason


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