68 lines
2.7 KiB
Diff
68 lines
2.7 KiB
Diff
From: Zheng Liu <gnehzuil.liu@gmail.com>
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Date: Sun, 29 Nov 2015 17:17:05 -0800
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Subject: [1/8] bcache: fix a livelock when we cause a huge number of cache
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misses
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Origin: https://git.kernel.org/cgit/linux/kernel/git/axboe/linux-block.git/commit?id=2ef9ccbfcb90cf84bdba320a571b18b05c41101b
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Subject : [PATCH v2] bcache: fix a livelock in btree lock
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Date : Wed, 25 Feb 2015 20:32:09 +0800 (02/25/2015 04:32:09 AM)
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This commit tries to fix a livelock in bcache. This livelock might
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happen when we causes a huge number of cache misses simultaneously.
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When we get a cache miss, bcache will execute the following path.
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->cached_dev_make_request()
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->cached_dev_read()
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->cached_lookup()
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->bch->btree_map_keys()
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->btree_root() <------------------------
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->bch_btree_map_keys_recurse() |
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->cache_lookup_fn() |
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->cached_dev_cache_miss() |
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->bch_btree_insert_check_key() -|
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[If btree->seq is not equal to seq + 1, we should return
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EINTR and traverse btree again.]
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In bch_btree_insert_check_key() function we first need to check upgrade
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flag (op->lock == -1), and when this flag is true we need to release
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read btree->lock and try to take write btree->lock. During taking and
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releasing this write lock, btree->seq will be monotone increased in
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order to prevent other threads modify this in cache miss (see btree.h:74).
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But if there are some cache misses caused by some requested, we could
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meet a livelock because btree->seq is always changed by others. Thus no
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one can make progress.
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This commit will try to take write btree->lock if it encounters a race
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when we traverse btree. Although it sacrifice the scalability but we
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can ensure that only one can modify the btree.
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Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
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Tested-by: Joshua Schmid <jschmid@suse.com>
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Tested-by: Eric Wheeler <bcache@linux.ewheeler.net>
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Cc: Joshua Schmid <jschmid@suse.com>
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Cc: Zhu Yanhai <zhu.yanhai@gmail.com>
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Cc: Kent Overstreet <kmo@daterainc.com>
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Cc: stable@vger.kernel.org
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Signed-off-by: Jens Axboe <axboe@fb.com>
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---
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drivers/md/bcache/btree.c | 4 +++-
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1 file changed, 3 insertions(+), 1 deletion(-)
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diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
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index 83392f8..4a1179c 100644
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--- a/drivers/md/bcache/btree.c
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+++ b/drivers/md/bcache/btree.c
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@@ -2162,8 +2162,10 @@ int bch_btree_insert_check_key(struct btree *b, struct btree_op *op,
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rw_lock(true, b, b->level);
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if (b->key.ptr[0] != btree_ptr ||
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- b->seq != seq + 1)
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+ b->seq != seq + 1) {
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+ op->lock = b->level;
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goto out;
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+ }
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}
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SET_KEY_PTRS(check_key, 1);
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