The kernel test robot reported two performance regressions
caused by recent patches.
Both appear to related to the global spinlock blocked_lock_lock
being taken more often.
This patch avoids taking that lock in the cases tested.
Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
Hi Jeff,
you might like to merge these back into the patches that introduced
the problem.
Or you might like me to re-send the series with these merged in,
in which case, please ask.
And a BIG thank-you to the kernel-test-robot team!!
Thanks,
NeilBrown
fs/locks.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/fs/locks.c b/fs/locks.c
index f456cd3d9d50..67519a43e27a 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -444,6 +444,13 @@ static void locks_move_blocks(struct file_lock *new, struct file_lock *fl)
{
struct file_lock *f;
+ /*
+ * As ctx->flc_lock is held, new requests cannot be added to
+ * ->fl_blocked_requests, so we don't need a lock to check if it
+ * is empty.
+ */
+ if (list_empty(&fl->fl_blocked_requests))
+ return;
spin_lock(&blocked_lock_lock);
list_splice_init(&fl->fl_blocked_requests, &new->fl_blocked_requests);
list_for_each_entry(f, &fl->fl_blocked_requests, fl_blocked_member)
@@ -749,6 +756,20 @@ int locks_delete_block(struct file_lock *waiter)
{
int status = -ENOENT;
+ /*
+ * If fl_blocker is NULL, it won't be set again as this thread
+ * "owns" the lock and is the only one that might try to claim
+ * the lock. So it is safe to test fl_blocker locklessly.
+ * Also if fl_blocker is NULL, this waiter is not listed on
+ * fl_blocked_requests for some lock, so no other request can
+ * be added to the list of fl_blocked_requests for this
+ * request. So if fl_blocker is NULL, it is safe to
+ * locklessly check if fl_blocked_requests is empty. If both
+ * of these checks succeed, there is no need to take the lock.
+ */
+ if (waiter->fl_blocker == NULL &&
+ list_empty(&waiter->fl_blocked_requests))
+ return status;
spin_lock(&blocked_lock_lock);
if (waiter->fl_blocker)
status = 0;
--
2.14.0.rc0.dirty
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