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Date: Fri, 31 May 2013 03:32:58 +0900
From: Greg KH <gregkh@linuxfoundation.org>
To: Rafael Aquini <aquini@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, hughd@google.com, shli@kernel.org,
        riel@redhat.com, lwoodman@redhat.com, kosaki.motohiro@jp.fujitsu.com,
        kamezawa.hiroyu@jp.fujitsu.com, stable@vger.kernel.org
Subject: Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock
 while waiting on discard I/O compeletion
Message-ID: <20130530183258.GA3453@kroah.com>
References: <2434dea05a7fda7e7ccf48f70124bd65f2556b2d.1369935749.git.aquini@redhat.com>
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On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> 
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---
>  mm/swap_state.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)


<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

</formletter>
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