Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

Jeff Layton
2020-03-17T15:59:29Z

On Tue, 2020-03-17 at 09:45 +1100, NeilBrown wrote:
> > +
> > + /*
> > + * Tell the world we're done with it - see comment at top
> > + * of this function
>
> This comment might be misleading. The world doesn't care.
> Only this thread cares where ->fl_blocker is NULL. We need the release
> semantics when some *other* thread sets fl_blocker to NULL, not when
> this thread does.
> I don't think we need to spell that out and I'm not against using
> store_release here, but locks_delete_block cannot race with itself, so
> referring to the comment at the top of this function is misleading.
>
> So:
> Reviewed-by: NeilBrown <neilb@suse.de>
>
> but I'm not totally happy with the comments.
>
>

Thanks Neil. We can clean up the comments before merge. How about this
revision to the earlier patch? I took the liberty of poaching your your
proposed verbiage:

------------------8<---------------------

From c9fbfae0ab615e20de0bdf1ae7b27591d602f577 Mon Sep 17 00:00:00 2001
From: Jeff Layton <jlayton@kernel.org>
Date: Mon, 16 Mar 2020 18:57:47 -0400
Subject: [PATCH] SQUASH: update with Neil's comments

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
fs/locks.c | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/fs/locks.c b/fs/locks.c
index eaf754ecdaa8..e74075b0e8ec 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -741,8 +741,9 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
wake_up(&waiter->fl_wait);

/*
- * Tell the world we're done with it - see comment at
- * top of locks_delete_block().
+ * The setting of fl_blocker to NULL marks the official "done"
+ * point in deleting a block. Paired with acquire at the top
+ * of locks_delete_block().
*/
smp_store_release(&waiter->fl_blocker, NULL);
}
@@ -761,11 +762,23 @@ int locks_delete_block(struct file_lock *waiter)
/*
* 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.
- * Because fl_blocker is explicitly set last during a delete, it's
- * safe to locklessly test to see if it's NULL. If it is, then we know
- * that no new locks can be inserted into its fl_blocked_requests list,
- * and we can therefore avoid doing anything further as long as that
- * list is empty.
+ *
+ * We use acquire/release to manage fl_blocker so that we can
+ * optimize away taking the blocked_lock_lock in many cases.
+ *
+ * The smp_load_acquire guarantees two things:
+ *
+ * 1/ that fl_blocked_requests can be tested locklessly. If something
+ * was recently added to that list it must have been in a locked region
+ * *before* the locked region when fl_blocker was set to NULL.
+ *
+ * 2/ that no other thread is accessing 'waiter', so it is safe to free
+ * it. __locks_wake_up_blocks is careful not to touch waiter after
+ * fl_blocker is released.
+ *
+ * If a lockless check of fl_blocker shows it to be NULL, we know that
+ * no new locks can be inserted into its fl_blocked_requests list, and
+ * can avoid doing anything further if the list is empty.
*/
if (!smp_load_acquire(&waiter->fl_blocker) &&
list_empty(&waiter->fl_blocked_requests))
@@ -778,8 +791,8 @@ int locks_delete_block(struct file_lock *waiter)
__locks_delete_block(waiter);

/*
- * Tell the world we're done with it - see comment at top
- * of this function
+ * The setting of fl_blocker to NULL marks the official "done" point in
+ * deleting a block. Paired with acquire at the top of this function.
*/
smp_store_release(&waiter->fl_blocker, NULL);
spin_unlock(&blocked_lock_lock);
--
2.24.1