| Message ID | 1454507914-28823-1-git-send-email-luis.henriques@canonical.com |
|---|---|
| State | New |
| Headers | show |
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c index 5ef445d2d81e..40a7a05e42ef 100644 --- a/drivers/md/bcache/btree.c +++ b/drivers/md/bcache/btree.c @@ -2150,8 +2150,10 @@ int bch_btree_insert_check_key(struct btree *b, struct btree_op *op, 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);
This is a note to let you know that I have just added a patch titled bcache: fix a livelock when we cause a huge number of cache misses to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree which can be found at: http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue This patch is scheduled to be released in version 3.16.7-ckt24. If you, or anyone else, feels it should not be added to this tree, please reply to this email. For more information about the 3.16.y-ckt tree, see https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable Thanks. -Luis ---8<------------------------------------------------------------ From dbec969a48d22b3a4a84ca136123267cedfc440c 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 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: Luis Henriques <luis.henriques@canonical.com> --- drivers/md/bcache/btree.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)