Commit cbab0e4e authored Jun 12, 2013 by Rafael Aquini Committed by Linus Torvalds Jun 12, 2013

swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O completion



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 waitqueue 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: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 24b8256a

mm/swap_state.c

+17 −1

Original line number	Diff line number	Diff line
		@@ -336,8 +336,24 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
		* Swap entry may have been freed since our caller observed it.
		*/
		err = swapcache_prepare(entry);
		if (err == -EEXIST) { /* seems racy */
		if (err == -EEXIST) {
		radix_tree_preload_end();
		/*
		* We might race against get_swap_page() and stumble
		* across a SWAP_HAS_CACHE swap_map entry whose page
		* has not been brought into the swapcache yet, while
		* the other end is scheduled away waiting on discard
		* I/O completion at scan_swap_map().
		*
		* In order to avoid turning this transitory state
		* into a permanent loop around this -EEXIST case
		* if !CONFIG_PREEMPT and the I/O completion happens
		* to be waiting on the CPU waitqueue where we are now
		* busy looping, we just conditionally invoke the
		* scheduler here, if there are some more important
		* tasks to run.
		*/
		cond_resched();
		continue;
		}
		if (err) { /* swp entry is obsolete ? */