On Tue, Dec 20, 2022 at 10:43:25PM -0500, Mathieu Desnoyers wrote: > On 2022-12-20 19:58, Frederic Weisbecker wrote: > > On Wed, Dec 21, 2022 at 01:49:57AM +0100, Frederic Weisbecker wrote: > > > On Tue, Dec 20, 2022 at 07:15:00PM -0500, Joel Fernandes wrote: > > > > On Tue, Dec 20, 2022 at 5:45 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote: > > > > Agreed about (1). > > > > > > > > > _ In (2), E pairs with the address-dependency between idx and lock_count. > > > > > > > > But that is not the only reason. If that was the only reason for (2), > > > > then there is an smp_mb() just before the next-scan post-flip before > > > > the lock counts are read. > > > > > > The post-flip barrier makes sure the new idx is visible on the next READER's > > > turn, but it doesn't protect against the fact that "READ idx then WRITE lock[idx]" > > > may appear unordered from the update side POV if there is no barrier between the > > > scan and the flip. > > > > > > If you remove the smp_mb() from the litmus test I sent, things explode. > > > > Or rather, look at it the other way, if there is no barrier between the lock > > scan and the index flip (E), then the index flip can appear to be written before the > > lock is read. Which means you may start activating the index before you finish > > reading it (at least it appears that way from the readers pont of view). > > Considering that you can have pre-existing readers from arbitrary index > appearing anywhere in the grace period (because a reader can fetch the > index and be preempted for an arbitrary amount of time before incrementing > the lock count), the grace period algorithm needs to deal with the fact that > a newcoming reader can appear in a given index either before or after the > flip. True but the number of preempted tasks is bound and there is a forward progress guarantee. > I don't see how flipping the index before or after loading the unlock/lock > values would break anything (except for unlikely counter overflow situations > as previously discussed). Forward progress guarantee. Thanks. > > Thanks, > > Mathieu > > -- > Mathieu Desnoyers > EfficiOS Inc. > https://www.efficios.com >
