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Message-ID: <1463592095.3369.10.camel@j-VirtualBox>
Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by
 WRITE_ONCE
From: Jason Low <jason.low2@hpe.com>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Waiman Long <Waiman.Long@hpe.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Dave Chinner <david@fromorbit.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>, jason.low2@hp.com
Date: Wed, 18 May 2016 10:21:35 -0700
In-Reply-To: <20160518140436.GA6273@linux-uzut.site>
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On Wed, 2016-05-18 at 07:04 -0700, Davidlohr Bueso wrote:
> 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.

It might be okay to leave out READ_ONCE() for reading rwsem->owner, but
couldn't we include it to at least document that we're performing a
"special" lockless read?