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From: Jeff Layton <jlayton@kernel.org>
To: NeilBrown <neilb@suse.de>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: yangerkun <yangerkun@huawei.com>,
	kernel test robot <rong.a.chen@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	lkp@lists.01.org, Bruce Fields <bfields@fieldses.org>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression
Date: Tue, 17 Mar 2020 11:59:24 -0400	[thread overview]
Message-ID: <46d2c16f48f1fd4ad28a85099c59ae95a9997740.camel@kernel.org> (raw)
In-Reply-To: <87k13jsyum.fsf@notabene.neil.brown.name>

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


  reply	other threads:[~2020-03-17 15:59 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-08 14:03 [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression kernel test robot
2020-03-09 14:36 ` Jeff Layton
2020-03-09 15:52   ` Linus Torvalds
2020-03-09 17:22     ` Jeff Layton
2020-03-09 19:09       ` Jeff Layton
2020-03-09 19:53         ` Jeff Layton
2020-03-09 21:42         ` NeilBrown
2020-03-09 21:58           ` Jeff Layton
2020-03-10  7:52             ` kernel test robot
2020-03-09 22:11           ` Jeff Layton
2020-03-10  3:24             ` yangerkun
2020-03-10  7:54               ` kernel test robot
2020-03-10 12:52               ` Jeff Layton
2020-03-10 14:18                 ` yangerkun
2020-03-10 15:06                   ` Jeff Layton
2020-03-10 17:27                 ` Jeff Layton
2020-03-10 21:01                   ` NeilBrown
2020-03-10 21:14                     ` Jeff Layton
2020-03-10 21:21                       ` NeilBrown
2020-03-10 21:47                         ` Linus Torvalds
2020-03-10 22:07                           ` Jeff Layton
2020-03-10 22:31                             ` Linus Torvalds
2020-03-11 22:22                               ` NeilBrown
2020-03-12  0:38                                 ` Linus Torvalds
2020-03-12  4:42                                   ` NeilBrown
2020-03-12 12:31                                     ` Jeff Layton
2020-03-12 22:19                                       ` NeilBrown
2020-03-14  1:11                                         ` Jeff Layton
2020-03-12 16:07                                     ` Linus Torvalds
2020-03-14  1:31                                       ` Jeff Layton
2020-03-14  2:31                                         ` NeilBrown
2020-03-14 15:58                                           ` Linus Torvalds
2020-03-15 13:54                                             ` Jeff Layton
2020-03-16  5:06                                               ` NeilBrown
2020-03-16 11:07                                                 ` Jeff Layton
2020-03-16 17:26                                                   ` Linus Torvalds
2020-03-17  1:41                                                     ` yangerkun
2020-03-17 14:05                                                       ` yangerkun
2020-03-17 16:07                                                         ` Jeff Layton
2020-03-18  1:09                                                           ` yangerkun
2020-03-19 17:51                                                     ` Jeff Layton
2020-03-19 19:23                                                       ` Linus Torvalds
2020-03-19 19:24                                                         ` Jeff Layton
2020-03-19 19:35                                                           ` Linus Torvalds
2020-03-19 20:10                                                             ` Jeff Layton
2020-03-16 22:45                                                   ` NeilBrown
2020-03-17 15:59                                                     ` Jeff Layton [this message]
2020-03-17 21:27                                                       ` NeilBrown
2020-03-18  5:12                                                   ` kernel test robot
2020-03-16  4:26                                             ` NeilBrown
2020-03-11  1:57                     ` yangerkun
2020-03-11 12:52                       ` Jeff Layton
2020-03-11 13:26                         ` yangerkun
2020-03-11 22:15                       ` NeilBrown
2020-03-10  7:50           ` kernel test robot

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