[patch 12/28] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O completion

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From: Rafael Aquini <aquini@xxxxxxxxxx>
Subject: 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@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/swap_state.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff -puN mm/swap_state.c~swap-avoid-read_swap_cache_async-race-to-deadlock-while-waiting-on-discard-i-o-completion mm/swap_state.c
--- a/mm/swap_state.c~swap-avoid-read_swap_cache_async-race-to-deadlock-while-waiting-on-discard-i-o-completion
+++ a/mm/swap_state.c
@@ -336,8 +336,24 @@ struct page *read_swap_cache_async(swp_e
 		 * 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 ? */
_
--
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