Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Subject | Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression | From | Jeff Layton <> | Date | Tue, 17 Mar 2020 11:59:24 -0400 |
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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:
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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
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