Zheng,
It should be 'op->lock = b->level', not 'op->lock = b->c->root->level
+ 1', otherwise we will stop all concurrency writes unconditionally in
the second round. Isn't it?
-zyh
2015-02-03 19:21 GMT+08:00 Joshua Schmid <jschmid@xxxxxxxx>:
> From: Zheng Liu <wenqing.lz@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Tested-by: Joshua Schmid <jschmid@xxxxxxxx>
> ---
> drivers/md/bcache/btree.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index 218f21a..f1c224f 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -2163,8 +2163,10 @@ 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->c->root->level + 1;
> goto out;
> + }
> }
>
> SET_KEY_PTRS(check_key, 1);
> --
> 2.1.2
>
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