From: NeilBrown <neilb@suse.de>
To: Jeff Layton <jlayton@kernel.org>,
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 09:45:37 +1100 [thread overview]
Message-ID: <87k13jsyum.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <ce48ed9e48eda3c0f27d2f417314bd00cb1a68db.camel@kernel.org>
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On Mon, Mar 16 2020, Jeff Layton wrote:
> @@ -740,6 +739,12 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
> waiter->fl_lmops->lm_notify(waiter);
> else
> wake_up(&waiter->fl_wait);
> +
> + /*
> + * Tell the world we're done with it - see comment at
> + * top of locks_delete_block().
> + */
> + smp_store_release(&waiter->fl_blocker, NULL);
> }
> }
>
> @@ -753,11 +758,30 @@ 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.
> + * 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.
I think it would be worth spelling out what the 'acquire' is needed
for. We seem to have a general policy of requiring comment to explain
the presence of barriers.
The 'acquire' on fl_blocker 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 (!smp_load_acquire(&waiter->fl_blocker) &&
> + list_empty(&waiter->fl_blocked_requests))
> + return status;
> +
> spin_lock(&blocked_lock_lock);
> if (waiter->fl_blocker)
> status = 0;
> __locks_wake_up_blocks(waiter);
> __locks_delete_block(waiter);
> +
> + /*
> + * 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,
NeilBrown
> + */
> + smp_store_release(&waiter->fl_blocker, NULL);
> spin_unlock(&blocked_lock_lock);
> return status;
> }
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next prev parent reply other threads:[~2020-03-16 22:45 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 [this message]
2020-03-17 15:59 ` Jeff Layton
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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