linux-kernel - Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE

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Message-ID: <20160518140436.GA6273@linux-uzut.site>
Date:	Wed, 18 May 2016 07:04:36 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by
 WRITE_ONCE()

On Tue, 17 May 2016, Waiman Long wrote:

>Without using WRITE_ONCE(), the compiler can potentially break a
>write into multiple smaller ones (store tearing). So a read from the
>same data by another task concurrently may return a partial result.
>This can result in a kernel crash if the data is a memory address
>that is being dereferenced.
>
>This patch changes all write to rwsem->owner to use WRITE_ONCE()
>to make sure that store tearing will not happen. READ_ONCE() may
>not be needed for rwsem->owner as long as the value is only used for
>comparison and not dereferencing.
>
>Signed-off-by: Waiman Long <Waiman.Long@....com>

Yes, ->owner can obviously be handled locklessly during optimistic
spinning.

Acked-by: Davidlohr Bueso <dave@...olabs.net>