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Date: Mon, 17 Aug 2015 16:22:21 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
Subject: Re: Potential data race in SyS_swapon
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To: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@suse.cz>, Johannes Weiner <hannes@cmpxchg.org>, Vladimir Davydov <vdavydov@parallels.com>, Hugh Dickins <hughd@google.com>, Miklos Szeredi <mszeredi@suse.cz>, Jason Low <jason.low2@hp.com>, Cesar Eduardo Barros <cesarb@cesarb.net>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dmitry Vyukov <dvyukov@google.com>, Kostya Serebryany <kcc@google.com>, Alexander Potapenko <glider@google.com>

On Fri, 7 Aug 2015, Andrey Konovalov wrote:
> 
> We are working on a dynamic data race detector for the Linux kernel
> called KernelThreadSanitizer (ktsan)
> (https://github.com/google/ktsan/wiki).
> 
> While running ktsan on the upstream revision 21bdb584af8c with trinity
> we got a few reports from SyS_swapon, here is one of them:
> 
> ==================================================================
> ThreadSanitizer: data-race in SyS_swapon
> 
> Read of size 8 by thread T307 (K7621):
>  [<     inlined    >] SyS_swapon+0x3c0/0x1850 SYSC_swapon mm/swapfile.c:2395
>  [<ffffffff812242c0>] SyS_swapon+0x3c0/0x1850 mm/swapfile.c:2345
>  [<ffffffff81e97c8a>] ia32_do_call+0x1b/0x25
> 
> Looks like the swap_lock should be taken when iterating through the
> swap_info array on lines 2392 - 2401.

Thanks for the report.  Actually, lines 2392 to 2401 just look redundant
to me: it looks as if claim_swapfile() should do all that's needed,
though in fact it doesn't quite.  I'll send akpm a patch and Cc you,
no need to retest since the offending lines just won't be there.

Hugh

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