linux-kernel - Re: [PATCH v2 31/32] sh: support a 2-byte smp_store

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Message-ID: <20160105232735.GC238@brightrain.aerifal.cx>
Date:	Tue, 5 Jan 2016 18:27:35 -0500
From:	Rich Felker <dalias@...c.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org
Subject: Re: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb

On Thu, Dec 31, 2015 at 09:09:47PM +0200, Michael S. Tsirkin wrote:
> At the moment, xchg on sh only supports 4 and 1 byte values, so using it
> from smp_store_mb means attempts to store a 2 byte value using this
> macro fail.
> 
> And happens to be exactly what virtio drivers want to do.
> 
> Check size and fall back to a slower, but safe, WRITE_ONCE+smp_mb.

Can you please do this for size 1 as well (i.e. all sizes != 4)? If
you check the source, the code for size-1 xchg in sh cmpxchg-llsc.h is
completely wrong and operates on a 32-bit object at the address passed
to it. This code is presently unused anyway and I plan to submit a
patch to remove the size 1 case.

Rich
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