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

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Message-ID: <20200219052329.GP23230@ZenIV.linux.org.uk>
Date:   Wed, 19 Feb 2020 05:23:29 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Qian Cai <cai@....pw>
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 Wed, Feb 19, 2020 at 12:08:40AM -0500, Qian Cai wrote:
> 
> 
> > 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().
> 

	I would suggest
* if some compiler does that, ask the persons responsible for that
"optimization" which flags should be used to disable it.
* if they fail to provide such, educate them regarding the
usefulness of their idea
* if that does not help, don't use the bloody piece of garbage.

	Again, is that pure theory (because I can't come up with
any reason why splitting a 32bit load would be any less legitimate
than doing the same to a 64bit one on a 64bit architecture),
or is there anything that really would pull that off?