From: Jeff Layton <jlayton@kernel.org>
To: "J. Bruce Fields" <bfields@fieldses.org>, NeilBrown <neilb@suse.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
Martin Wilck <mwilck@suse.de>,
linux-fsdevel@vger.kernel.org,
Frank Filz <ffilzlnx@mindspring.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups
Date: Sat, 11 Aug 2018 07:51:13 -0400 [thread overview]
Message-ID: <f1b9ca11a26eafbeb348bbfdf12d2ccdd43dc3c9.camel@kernel.org> (raw)
In-Reply-To: <20180810002922.GA3915@fieldses.org>
On Thu, 2018-08-09 at 20:29 -0400, J. Bruce Fields wrote:
> On Fri, Aug 10, 2018 at 08:12:43AM +1000, NeilBrown wrote:
> > On Thu, Aug 09 2018, J. Bruce Fields wrote:
> >
> > > I think there's also a problem with multiple tasks sharing the same
> > > lock owner.
> > >
> > > So, all locks are exclusive locks for the same range. We have four
> > > tasks. Tasks 1 and 4 share the same owner, the others' owners are
> > > distinct.
> > >
> > > - Task 1 gets a lock.
> > > - Task 2 gets a conflicting lock.
> > > - Task 3 gets another conflicting lock. So now we the tree is
> > > 3->2->1.
> > > - Task 1's lock is released.
> > > - Before task 2 is scheduled, task 4 acquires a new lock.
> > > - Task 2 waits on task 4's lock, we now have
> > > 3->2->4.
> > >
> > > Task 3 shouldn't be waiting--the lock it's requesting has the same owner
> > > as the lock task 4 holds--but we fail to wake up task 3.
> >
> > So task 1 and task 4 are threads in the one process - OK.
> > Tasks 2 and 3 are threads in two other processes.
> >
> > So 2 and 3 conflict with either 1 or 4 equally - why should task 3 be
> > woken?
> >
> > I suspect you got the numbers bit mixed up,
>
> Whoops.
>
> > but in any case, the "conflict()" function that is passed around takes
> > ownership into account when assessing if one lock conflicts with
> > another.
>
> Right, I know, but, let me try again:
>
> All locks are exclusive locks for the same range. Only tasks 3 and 4
> share the the same owner.
>
> - Task 1 gets a lock.
> - Task 2 requests a conflicting lock, so we have 2->1.
> - Task 3 requests a conflicting lock, so we have 3->2->1.
> - Task 1 unlocks. We wake up task 2, but it isn't scheduled yet.
> - Task 4 gets a new lock.
> - Task 2 runs, discovers the conflict, and waits. Now we have:
> 3->2->4.
>
> There is no conflict between the lock 3 requested and the lock 4 holds,
> but 3 is not woken up.
>
> This is another version of the first problem: there's information we
> need (the owners of the waiting locks in the tree) that we can't
> determine just from looking at the root of the tree.
>
> I'm not sure what to do about that.
>
Is this still a problem in the v2 set?
wake_non_conflicts walks the whole tree of requests that were blocked on
it, so a. After task 2 discovers the conflict, it should wake any of its
children that don't conflict. So in that last step, task 3 would be
awoken before task 2 goes back to sleep.
--
Jeff Layton <jlayton@kernel.org>
next prev parent reply other threads:[~2018-08-11 11:51 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-09 2:04 [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups NeilBrown
2018-08-09 2:04 ` [PATCH 1/5] fs/locks: rename some lists and pointers NeilBrown
2018-08-09 2:04 ` [PATCH 2/5] fs/locks: allow a lock request to block other requests NeilBrown
2018-08-09 2:04 ` [PATCH 3/5] fs/locks: change all *_conflict() functions to return a new enum NeilBrown
2018-08-09 11:09 ` Jeff Layton
2018-08-09 13:09 ` J. Bruce Fields
2018-08-09 23:40 ` NeilBrown
2018-08-10 0:56 ` J. Bruce Fields
2018-08-09 2:04 ` [PATCH 4/5] fs/locks: split out __locks_wake_one() NeilBrown
2018-08-09 2:04 ` [PATCH 5/5] fs/locks: create a tree of dependent requests NeilBrown
2018-08-09 11:17 ` Jeff Layton
2018-08-09 23:25 ` NeilBrown
2018-08-09 14:13 ` J. Bruce Fields
2018-08-09 22:19 ` NeilBrown
2018-08-10 0:36 ` J. Bruce Fields
2018-08-09 17:32 ` [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups J. Bruce Fields
2018-08-09 22:12 ` NeilBrown
2018-08-10 0:29 ` J. Bruce Fields
2018-08-10 1:50 ` NeilBrown
2018-08-10 2:52 ` J. Bruce Fields
2018-08-10 3:17 ` NeilBrown
2018-08-10 15:47 ` J. Bruce Fields
2018-08-11 11:56 ` Jeff Layton
2018-08-11 12:35 ` J. Bruce Fields
2018-08-11 11:51 ` Jeff Layton [this message]
2018-08-11 12:21 ` J. Bruce Fields
2018-08-11 13:15 ` Jeff Layton
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=f1b9ca11a26eafbeb348bbfdf12d2ccdd43dc3c9.camel@kernel.org \
--to=jlayton@kernel.org \
--cc=bfields@fieldses.org \
--cc=ffilzlnx@mindspring.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mwilck@suse.de \
--cc=neilb@suse.com \
--cc=viro@zeniv.linux.org.uk \
/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).