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From: "J. Bruce Fields" <bfields@fieldses.org>
To: NeilBrown <neilb@suse.com>
Cc: Jeff Layton <jlayton@kernel.org>,
	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: Thu, 9 Aug 2018 20:29:22 -0400
Message-ID: <20180810002922.GA3915@fieldses.org> (raw)
In-Reply-To: <87lg9frxyc.fsf@notabene.neil.brown.name>

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.

--b.

  reply index

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-09  2:04 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 [this message]
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
2018-08-11 12:21         ` J. Bruce Fields
2018-08-11 13:15           ` Jeff Layton

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