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>

1 files changed, 17 insertions, 1 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b3d40dcf3624..f24ab0dff554 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -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 ? */