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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH] fs: fix a data race in i_size_write/i_size_read
Date:   Wed, 19 Feb 2020 00:08:40 -0500
Message-Id: <EDCBB5B9-DEE4-4D4B-B2F4-F63179977D6C@lca.pw>
References: <20200219045228.GO23230@ZenIV.linux.org.uk>
Cc:     hch@infradead.org, darrick.wong@oracle.com, elver@google.com,
        linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <20200219045228.GO23230@ZenIV.linux.org.uk>
To:     Al Viro <viro@zeniv.linux.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 18, 2020, at 11:52 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>=20
> If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end u=
p
> being split, the kernel is FUBAR anyway.  Details, please - how could that=

> end up happening?

My understanding is the compiler might decide to split the load into saying t=
wo 4-byte loads. Then, we might have,

Load1
Store
Load2

where the load value could be a garbage. Also, Marco (the KCSAN maintainer) w=
ho knew more of compiler than me mentioned that there is no guarantee that t=
he store will not be split either. Thus, the WRITE_ONCE().