Patch "bcache: fix a livelock when we cause a huge number of cache misses" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    bcache: fix a livelock when we cause a huge number of cache misses

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     bcache-fix-a-livelock-when-we-cause-a-huge-number-of-cache-misses.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 2ef9ccbfcb90cf84bdba320a571b18b05c41101b Mon Sep 17 00:00:00 2001
From: Zheng Liu <gnehzuil.liu@gmail.com>
Date: Sun, 29 Nov 2015 17:17:05 -0800
Subject: bcache: fix a livelock when we cause a huge number of cache misses

From: Zheng Liu <gnehzuil.liu@gmail.com>

commit 2ef9ccbfcb90cf84bdba320a571b18b05c41101b upstream.

Subject :	[PATCH v2] bcache: fix a livelock in btree lock
Date :	Wed, 25 Feb 2015 20:32:09 +0800 (02/25/2015 04:32:09 AM)

This commit tries to fix a livelock in bcache.  This livelock might
happen when we causes a huge number of cache misses simultaneously.

When we get a cache miss, bcache will execute the following path.

->cached_dev_make_request()
  ->cached_dev_read()
    ->cached_lookup()
      ->bch->btree_map_keys()
        ->btree_root()  <------------------------
          ->bch_btree_map_keys_recurse()        |
            ->cache_lookup_fn()                 |
              ->cached_dev_cache_miss()         |
                ->bch_btree_insert_check_key() -|
                  [If btree->seq is not equal to seq + 1, we should return
                   EINTR and traverse btree again.]

In bch_btree_insert_check_key() function we first need to check upgrade
flag (op->lock == -1), and when this flag is true we need to release
read btree->lock and try to take write btree->lock.  During taking and
releasing this write lock, btree->seq will be monotone increased in
order to prevent other threads modify this in cache miss (see btree.h:74).
But if there are some cache misses caused by some requested, we could
meet a livelock because btree->seq is always changed by others.  Thus no
one can make progress.

This commit will try to take write btree->lock if it encounters a race
when we traverse btree.  Although it sacrifice the scalability but we
can ensure that only one can modify the btree.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Tested-by: Joshua Schmid <jschmid@suse.com>
Tested-by: Eric Wheeler <bcache@linux.ewheeler.net>
Cc: Joshua Schmid <jschmid@suse.com>
Cc: Zhu Yanhai <zhu.yanhai@gmail.com>
Cc: Kent Overstreet <kmo@daterainc.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/md/bcache/btree.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -2162,8 +2162,10 @@ int bch_btree_insert_check_key(struct bt
 		rw_lock(true, b, b->level);
 
 		if (b->key.ptr[0] != btree_ptr ||
-		    b->seq != seq + 1)
+                   b->seq != seq + 1) {
+                       op->lock = b->level;
 			goto out;
+               }
 	}
 
 	SET_KEY_PTRS(check_key, 1);


Patches currently in stable-queue which might be from gnehzuil.liu@gmail.com are

queue-4.4/bcache-fix-a-livelock-when-we-cause-a-huge-number-of-cache-misses.patch