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SubjectRe: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups
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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>

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