Messages in this thread | | | From | Josh Don <> | Date | Thu, 29 Apr 2021 13:11:54 -0700 | Subject | Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock |
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On Wed, Apr 28, 2021 at 2:13 AM 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.
Ah yes, makes sense. Did you want to use preempt_enable_no_resched() at all then? No chance of preempt_count() being 1 at the point of enable, as you comment below.
> > 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 )
No worries, hope it's a mild one.
> How's this then?
Looks good to me (other than the synchronize_sched()->synchronize_rcu()).
For these locking patches, Reviewed-by: Josh Don <joshdon@google.com>
I'll see if I can repro that lockup.
> --- > 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(); > __sched_core_flip(true); > sched_core_assert_empty(); > } > @@ -449,16 +453,23 @@ void raw_spin_rq_lock_nested(struct rq *rq, int subclass) > { > raw_spinlock_t *lock; > > + /* Matches synchronize_sched() in __sched_core_enabled() */ > + preempt_disable(); > if (sched_core_disabled()) { > raw_spin_lock_nested(&rq->__lock, subclass); > + /* preempt-count *MUST* be > 1 */ > + preempt_enable_no_resched(); > return; > } > > for (;;) { > lock = __rq_lockp(rq); > raw_spin_lock_nested(lock, subclass); > - if (likely(lock == __rq_lockp(rq))) > + if (likely(lock == __rq_lockp(rq))) { > + /* preempt-count *MUST* be > 1 */ > + preempt_enable_no_resched(); > return; > + } > raw_spin_unlock(lock); > } > } > @@ -468,14 +479,21 @@ bool raw_spin_rq_trylock(struct rq *rq) > raw_spinlock_t *lock; > bool ret; > > - if (sched_core_disabled()) > - return raw_spin_trylock(&rq->__lock); > + /* Matches synchronize_sched() in __sched_core_enabled() */ > + preempt_disable(); > + if (sched_core_disabled()) { > + ret = raw_spin_trylock(&rq->__lock); > + preempt_enable(); > + return ret; > + } > > for (;;) { > lock = __rq_lockp(rq); > ret = raw_spin_trylock(lock); > - if (!ret || (likely(lock == __rq_lockp(rq)))) > + if (!ret || (likely(lock == __rq_lockp(rq)))) { > + preempt_enable(); > return ret; > + } > raw_spin_unlock(lock); > } > } > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 6a905fe19eef..c9a52231d58a 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -2568,11 +2568,12 @@ static inline void double_rq_unlock(struct rq *rq1, struct rq *rq2) > __releases(rq1->lock) > __releases(rq2->lock) > { > - raw_spin_rq_unlock(rq1); > if (__rq_lockp(rq1) != __rq_lockp(rq2)) > raw_spin_rq_unlock(rq2); > else > __release(rq2->lock); > + > + raw_spin_rq_unlock(rq1); > } > > extern void set_rq_online (struct rq *rq);
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