linux-kernel - Re: [PATCH] locking/mutex: Set and clear owner using WRITE

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Date:	Mon, 23 May 2016 14:31:23 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Jason Low <jason.low2@....com>
Cc:	jason.low2@...com, Waiman Long <waiman.long@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Terry Rudd <terry.rudd@....com>,
	Scott J Norton <scott.norton@....com>
Subject: Re: [PATCH] locking/mutex: Set and clear owner using WRITE_ONCE()

On Mon, 23 May 2016, Jason Low wrote:

>On Fri, 2016-05-20 at 18:00 -0700, Davidlohr Bueso wrote:
>> On Fri, 20 May 2016, Waiman Long wrote:
>>
>> >I think mutex-debug.h also needs similar changes for completeness.
>>
>> Maybe, but given that with debug the wait_lock is unavoidable, doesn't
>> this send the wrong message?
>
>The mutex_set_owner() and mutex_clear_owner() can still get called in
>the fastpath without the wait_lock for the debug case too correct?

The fastpath is a nop, see mutex-null:

#define __mutex_fastpath_lock(count, fail_fn)		fail_fn(count)

In anycase, if there were a fastpath, that would at least have an
ACQUIRE barrier (unless you set ->owner it before it or something
weird). But this isn't the case, so debug should be safe afaict.

Thanks,
Davidlohr