[ 3.8.y.z extended stable ] Patch "swap: avoid read_swap_cache_async() race to deadlock while waiting on" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    swap: avoid read_swap_cache_async() race to deadlock while waiting on

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 8e51f2012e0023450abbba1ea61416687bb5cf4c Mon Sep 17 00:00:00 2001
From: Rafael Aquini <aquini at redhat.com>
Date: Wed, 12 Jun 2013 14:04:49 -0700
Subject: 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 at redhat.com>
Acked-by: Johannes Weiner <hannes at cmpxchg.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd at google.com>
Cc: Shaohua Li <shli at kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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) {
 			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 ? */
--
1.8.1.2