Messages in this thread Patch in this message | | | Date | Wed, 28 Apr 2021 11:13:33 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock |
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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(); __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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