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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