Messages in this thread | | | Date | Sat, 11 Aug 2018 08:21:50 -0400 | From | "J. Bruce Fields" <> | Subject | Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups |
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On Sat, Aug 11, 2018 at 07:51:13AM -0400, Jeff Layton wrote: > 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,
Not in the FL_TRANSITIVE_CONFLICT case, which is the case here.
--b.
> 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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