Messages in this thread | | | From | Eric Dumazet <> | Date | Fri, 31 Jan 2020 09:06:27 -0800 | Subject | Re: Confused about hlist_unhashed_lockless() |
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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: > > > I just ran into c54a2744497d ("list: Add hlist_unhashed_lockless()") > > > but I'm a bit confused about what it's trying to achieve. It also seems > > > to have been merged without any callers (even in -next) -- was that > > > intentional? > > > > > > My main source of confusion is the lack of memory barriers. For example, > > > if you look at the following pair of functions: > > > > > > > > > static inline int hlist_unhashed_lockless(const struct hlist_node *h) > > > { > > > return !READ_ONCE(h->pprev); > > > } > > > > > > static inline void hlist_add_before(struct hlist_node *n, > > > struct hlist_node *next) > > > { > > > WRITE_ONCE(n->pprev, next->pprev); > > > WRITE_ONCE(n->next, next); > > > WRITE_ONCE(next->pprev, &n->next); > > > WRITE_ONCE(*(n->pprev), n); > > > } > > > > > > > > > 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.
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; }
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