Messages in this thread | | | From | Aubrey Li <> | Date | Wed, 28 Apr 2021 18:35:36 +0800 | Subject | Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock |
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On Wed, Apr 28, 2021 at 5:14 PM Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Apr 27, 2021 at 04:30:02PM -0700, Josh Don wrote: > > > Also, did you mean to have a preempt_enable_no_resched() rather than > > prempt_enable() in raw_spin_rq_trylock? > > No, trylock really needs to be preempt_enable(), because it can have > failed, at which point it will not have incremented the preemption count > and our decrement can hit 0, at which point we really should reschedule. > > > I went over the rq_lockp stuff again after Don's reported lockup. Most > > uses are safe due to already holding an rq lock. However, > > double_rq_unlock() is prone to race: > > > > double_rq_unlock(rq1, rq2): > > /* Initial state: core sched enabled, and rq1 and rq2 are smt > > siblings. So, double_rq_lock(rq1, rq2) only took a single rq lock */ > > raw_spin_rq_unlock(rq1); > > /* now not holding any rq lock */ > > /* sched core disabled. Now __rq_lockp(rq1) != __rq_lockp(rq2), so we > > falsely unlock rq2 */ > > if (__rq_lockp(rq1) != __rq_lockp(rq2)) > > raw_spin_rq_unlock(rq2); > > else > > __release(rq2->lock); > > > > Instead we can cache __rq_lockp(rq1) and __rq_lockp(rq2) before > > releasing the lock, in order to prevent this. FWIW I think it is > > likely that Don is seeing a different issue. > > Ah, indeed so.. rq_lockp() could do with an assertion, not sure how to > sanely do that. Anyway, double_rq_unlock() is simple enough to fix, we > can simply flip the unlock()s. > > ( I'm suffering a cold and am really quite slow atm ) > > How's this then? > > --- > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index f732642e3e09..3a534c0c1c46 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -290,6 +290,10 @@ static void sched_core_assert_empty(void) > static void __sched_core_enable(void) > { > static_branch_enable(&__sched_core_enabled); > + /* > + * Ensure raw_spin_rq_*lock*() have completed before flipping. > + */ > + synchronize_sched();
synchronize_sched() seems no longer exist...
Thanks, -Aubrey
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