diff options
authorRafael Aquini <>2013-06-12 14:04:49 -0700
committerGreg Kroah-Hartman <>2013-06-20 11:58:45 -0700
commitc09c35b2ae5ea7f62b0fd5369935b8e6af25e9cd (patch)
parent508056cc78435675c3f77dbb8de03f5b5ebe7d17 (diff)
swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O completion
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 <> Acked-by: Johannes Weiner <> Acked-by: KOSAKI Motohiro <> Acked-by: Hugh Dickins <> Cc: Shaohua Li <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <> Signed-off-by: Greg Kroah-Hartman <>
1 files changed, 17 insertions, 1 deletions
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 4c5ff7f284d..1fa92206829 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -313,8 +313,24 @@ 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 ? */