Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

On Thu, May 30, 2013 at 2:05 PM, Rafael Aquini <aquini@redhat.com> 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>

Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

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