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bcache: add comments for mutex_lock(&b->write_lock) 

When accessing or modifying BTREE_NODE_dirty bit, it is not always
necessary to acquire b->write_lock. In bch_btree_cache_free() and
mca_reap() acquiring b->write_lock is necessary, and this patch adds
comments to explain why mutex_lock(&b->write_lock) is necessary for
checking or clearing BTREE_NODE_dirty bit there.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Coly Li 2019-06-28 19:59:56 +08:00 committed by Jens Axboe
parent e5ec5f4765
commit 41508bb7d4
1 changed files with 10 additions and 0 deletions
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10
drivers/md/bcache/btree.c
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@ -659,6 +659,11 @@ static int mca_reap(struct btree *b, unsigned int min_order, bool flush)
up(&b->io_mutex);
}
/*
* BTREE_NODE_dirty might be cleared in btree_flush_btree() by
* __bch_btree_node_write(). To avoid an extra flush, acquire
* b->write_lock before checking BTREE_NODE_dirty bit.
*/
mutex_lock(&b->write_lock);
if (btree_node_dirty(b))
__bch_btree_node_write(b, &cl);
@ -782,6 +787,11 @@ void bch_btree_cache_free(struct cache_set *c)
while (!list_empty(&c->btree_cache)) {
b = list_first_entry(&c->btree_cache, struct btree, list);
/*
* This function is called by cache_set_free(), no I/O
* request on cache now, it is unnecessary to acquire
* b->write_lock before clearing BTREE_NODE_dirty anymore.
*/
if (btree_node_dirty(b)) {
btree_complete_write(b, btree_current_write(b));
clear_bit(BTREE_NODE_dirty, &b->flags);