linux-kernel - Re: [PATCH] fs: fix a data race in i_size_write/i_size

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Message-Id: <EDCBB5B9-DEE4-4D4B-B2F4-F63179977D6C@lca.pw>
Date:   Wed, 19 Feb 2020 00:08:40 -0500
From:   Qian Cai <cai@....pw>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     hch@...radead.org, darrick.wong@...cle.com, elver@...gle.com,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: fix a data race in i_size_write/i_size_read

> On Feb 18, 2020, at 11:52 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> 
> If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end up
> 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 two 4-byte loads. Then, we might have,

Load1
Store
Load2

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