btrfs: merge blocking_writers branches in btrfs_tree_read_lock (f5c2a525) · Commits · 戴 / test

There are two ifs that use eb::blocking_writers. As this is a variable modified inside and outside of locks, we could minimize number of accesses to avoid problems with getting different results at different times. The access here is locked so this can only race with btrfs_tree_unlock that sets blocking_writers to 0 without lock and unsets the lock owner. The first branch is taken only if the same thread already holds the lock, the second if checks for blocking writers. Here we'd either unlock and wait, or proceed. Both are valid states of the locking protocol. Reviewed-by:

Johannes Thumshirn <jthumshirn@suse.de> Signed-off-by:

David Sterba <dsterba@suse.com>

fs/btrfs/locking.c

+14 −13

Original line number	Diff line number	Diff line
		@@ -128,12 +128,14 @@ again:
		read_lock(&eb->lock);
		BUG_ON(eb->blocking_writers == 0 &&
		current->pid == eb->lock_owner);
		if (eb->blocking_writers && current->pid == eb->lock_owner) {
		if (eb->blocking_writers) {
		if (current->pid == eb->lock_owner) {
		/*
		* This extent is already write-locked by our thread. We allow
		* an additional read lock to be added because it's for the same
		* thread. btrfs_find_all_roots() depends on this as it may be
		* called on a partly (write-)locked tree.
		* This extent is already write-locked by our thread.
		* We allow an additional read lock to be added because
		* it's for the same thread. btrfs_find_all_roots()
		* depends on this as it may be called on a partly
		* (write-)locked tree.
		*/
		BUG_ON(eb->lock_nested);
		eb->lock_nested = true;
		@@ -141,7 +143,6 @@ again:
		trace_btrfs_tree_read_lock(eb, start_ns);
		return;
		}
		if (eb->blocking_writers) {
		read_unlock(&eb->lock);
		wait_event(eb->write_lock_wq,
		eb->blocking_writers == 0);

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