Excerpts from Linus Torvalds's message of June 30, 2020 2:36 am: > On Mon, Jun 29, 2020 at 6:16 AM Nicholas Piggin <npig...@gmail.com> wrote: >> >> No, ignore this part (which you explained well it was just a thinko, >> and your patch of course would not have worked if this was the case): >> the exclusive wake up doesn't get lost if schedule() was called because >> state goes back to running regardless of what woke it. > > Right. > > The normal pattern for a wait-loop is > > - add yourself to the wait-queue and set yourself "sleeping" with a > memory barrier. > > - test for the condition, exit if ok > > - go to sleep > > and that pattern doesn't have the race. > > The other common pattern is to check for the "do I need to sleep at > all" at the *top* of the function, long before you bother with any > wait-queues at all. This code does that odd "let's check in the middle > if we need to sleep" instead, which then caused the bug. > > So we had an odd setup because of three different wait conditions that > had different rules for what they could/should do before sleeping, and > then not sleeping reliably at all. > > We could also fix it by just splitting out the three cases into their > own wait routines that match the normal pattern. The bug really > happened because that wait-loop is doing things such an odd way due to > all the different cases.. > > I actually think it would be a lot more readable if it was three > different cases instead of trying to be one "common" routine. > > The *common* parts is the special PG_locked logic at the top, and the > thrashing/delayacct code at the bottom. > > And *that* could be a true common helper, but the wait loop (which > isn't even a loop for the DROP case) are fundamentally different and > probably should be separate functions. > > So I think my "one-liner" fixes the problem, but I'd certainly be open > to somebody cleaning this up properly.
I think it does. I would rather my patch which doesn't add a barrier to the "fast" path though (by the time we get here I think the fast path is that we are sleeping on the page, doing the IO). Alternatively (although this still adds more branching than necessary at least it doesn't have a barrier for the sleeping case), you could set the running state as an else case if you don't schedule. Thanks, Nick
