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locks: avoid taking global lock if possible when waking up blocked waiters

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Since we always hold the i_lock when inserting a new waiter onto the
fl_block list, we can avoid taking the global lock at all if we find
that it's empty when we go to wake up blocked waiters.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Jeff Layton 2013-06-21 08:58:16 -04:00 committed by Al Viro
parent 1c8c601a8c
commit 4e8c765d38
1 changed files with 14 additions and 1 deletions
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15
fs/locks.c
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@ -548,7 +548,10 @@ static void locks_delete_block(struct file_lock *waiter)
* the order they blocked. The documentation doesn't require this but
* it seems like the reasonable thing to do.
*
* Must be called with file_lock_lock held!
* Must be called with both the i_lock and file_lock_lock held. The fl_block
* list itself is protected by the file_lock_list, but by ensuring that the
* i_lock is also held on insertions we can avoid taking the file_lock_lock
* in some cases when we see that the fl_block list is empty.
*/
static void __locks_insert_block(struct file_lock *blocker,
struct file_lock *waiter)
@ -576,6 +579,16 @@ static void locks_insert_block(struct file_lock *blocker,
*/
static void locks_wake_up_blocks(struct file_lock *blocker)
{
/*
* Avoid taking global lock if list is empty. This is safe since new
* blocked requests are only added to the list under the i_lock, and
* the i_lock is always held here. Note that removal from the fl_block
* list does not require the i_lock, so we must recheck list_empty()
* after acquiring the file_lock_lock.
*/
if (list_empty(&blocker->fl_block))
return;
spin_lock(&file_lock_lock);
while (!list_empty(&blocker->fl_block)) {
struct file_lock *waiter;