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
next prev parent 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=46d2c16f48f1fd4ad28a85099c59ae95a9997740.camel@kernel.org \
--to=jlayton@kernel.org \
--cc=bfields@fieldses.org \
--cc=linux-kernel@vger.kernel.org \
--cc=lkp@lists.01.org \
--cc=neilb@suse.de \
--cc=rong.a.chen@intel.com \
--cc=torvalds@linux-foundation.org \
--cc=viro@zeniv.linux.org.uk \
--cc=yangerkun@huawei.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).