[PATCH 79/84] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O completion

From: Kamal Mostafa
Date: Mon Jun 17 2013 - 13:38:40 EST -stable review patch. If anyone has any objections, please let me know.


From: Rafael Aquini <aquini@xxxxxxxxxx>

commit cbab0e4eec299e9059199ebe6daf48730be46d2b upstream.

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>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
mm/swap_state.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 0cb36fb..f854fbd 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -314,8 +314,24 @@ 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) {
+ /*
+ * 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();
if (err) { /* swp entry is obsolete ? */

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