Messages in this thread | | | Date | Fri, 31 Jan 2020 17:20:58 +0000 | From | Will Deacon <> | Subject | Re: Confused about hlist_unhashed_lockless() |
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On Fri, Jan 31, 2020 at 09:06:27AM -0800, Eric Dumazet wrote: > On Fri, Jan 31, 2020 at 8:57 AM Will Deacon <will@kernel.org> wrote: > > On Fri, Jan 31, 2020 at 08:48:05AM -0800, Eric Dumazet wrote: > > > On Fri, Jan 31, 2020 at 8:43 AM Will Deacon <will@kernel.org> wrote: > > > > Then running these two concurrently on the same node means that > > > > hlist_unhashed_lockless() doesn't really tell you anything about whether > > > > or not the node is reachable in the list (i.e. there is another node > > > > with a next pointer pointing to it). In other words, I think all of > > > > these outcomes are permitted: > > > > > > > > hlist_unhashed_lockless(n) n reachable in list > > > > 0 0 (No reordering) > > > > 0 1 (No reordering) > > > > 1 0 (No reordering) > > > > 1 1 (Reorder first and last WRITE_ONCEs) > > > > > > > > So I must be missing some details about the use-case here. Please could > > > > you enlighten me? The RCU implementation permits only the first three > > > > outcomes afaict, why not use that and leave non-RCU hlist as it was? > > > > > > > > > > I guess the following has been lost : > > > > Thanks, although... > > > > > Author: Eric Dumazet <edumazet@google.com> > > > Date: Thu Nov 7 11:23:14 2019 -0800 > > > > > > timer: use hlist_unhashed_lockless() in timer_pending() > > > > > > timer_pending() is mostly used in lockless contexts. > > > > ... my point above still stands: the value returned by > > hlist_unhashed_lockless() doesn't tell you anything about whether or > > not the timer is reachable in the hlist or not. The comment above > > timer_pending() also states that: > > > > | Callers must ensure serialization wrt. other operations done to > > | this timer, e.g. interrupt contexts, or other CPUs on SMP. > > > > If that is intended to preclude list operations, shouldn't we use an > > RCU hlist instead of throwing {READ,WRITE}_ONCE() at the problem to > > shut the sanitiser up without actually fixing anything? :( > > > Sorry, but timer_pending() requires no serialization.
Then we should update the comment!
Without serialisation, timer_pending() as currently implemented does not reliably tell you whether the timer is in the hlist. Is that not a problem? Using an RCU hlist does not introduce serialisation, but does at least rule out the case where timer_pending() returns false for a timer that /is/ reachable in the list by another CPU.
> The only thing we need is a READ_ONCE() so that compiler is not allowed > to optimize out stuff like > > loop() { > if (timer_pending()) > something;
If that was the case, then you wouldn't need to touch hlist_add_before() at all so there's got to be more to it than that or we can revert that part of the patch.
Will
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