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

[Rafael Aquini <aquini@redhat.com>]

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(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b3d40dc..9ad9e3b 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -336,8 +336,20 @@ 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.
+			 * In order to avoid turning this transitory state
+			 * into a permanent loop around this -EEXIST case,
+			 * lets just conditionally invoke the scheduler,
+			 * if there are some more important tasks to run.
+			 */
+			cond_resched();
 			continue;
 		}
 		if (err) {		/* swp entry is obsolete ? */
-- 
1.8.1.4

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