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Date:   Wed, 28 Apr 2021 11:13:33 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Don <joshdon@google.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        "Hyser,Chris" <chris.hyser@oracle.com>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, dhiatt@digitalocean.com
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);