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authorRafael Aquini <aquini@redhat.com>2013-06-12 14:04:49 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-06-20 11:58:45 -0700
commitc09c35b2ae5ea7f62b0fd5369935b8e6af25e9cd (patch)
tree3899916b5478074de32197baade2e42f19d79ec8
parent508056cc78435675c3f77dbb8de03f5b5ebe7d17 (diff)
downloadlinux-linaro-stable-c09c35b2ae5ea7f62b0fd5369935b8e6af25e9cd.tar.gz
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 <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> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--mm/swap_state.c18
1 files changed, 17 insertions, 1 deletions
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 4c5ff7f284d9..1fa92206829a 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) {
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 ? */