| From | Greg Kroah-Hartman <> | Subject | [PATCH 3.14 006/130] bcache: fix a livelock when we cause a huge number of cache misses | Date | Tue, 1 Mar 2016 15:44:17 -0800 |
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3.14-stable review patch. If anyone has any objections, please let me know.
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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 @@ -2037,8 +2037,10 @@ 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);
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