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From:   Sultan Alsawaf <sultan@kerneltoast.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org,
        Sultan Alsawaf <sultan@kerneltoast.com>
Subject: [PATCH 1/2] locking/mutex: Don't hog RCU read lock while optimistically spinning
Date:   Fri,  7 Aug 2020 12:16:35 -0700
Message-Id: <20200807191636.75045-1-sultan@kerneltoast.com>
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From: Sultan Alsawaf <sultan@kerneltoast.com>

There's no reason to hold an RCU read lock the entire time while
optimistically spinning for a mutex lock. This can needlessly lengthen
RCU grace periods and slow down synchronize_rcu() when it doesn't brute
force the RCU grace period via rcupdate.rcu_expedited=1.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
 kernel/locking/mutex.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 5352ce50a97e..cc5676712458 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -552,21 +552,31 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 {
 	bool ret = true;
 
-	rcu_read_lock();
-	while (__mutex_owner(lock) == owner) {
+	for (;;) {
+		unsigned int cpu;
+		bool same_owner;
+
 		/*
-		 * Ensure we emit the owner->on_cpu, dereference _after_
-		 * checking lock->owner still matches owner. If that fails,
+		 * Ensure lock->owner still matches owner. If that fails,
 		 * owner might point to freed memory. If it still matches,
 		 * the rcu_read_lock() ensures the memory stays valid.
 		 */
-		barrier();
+		rcu_read_lock();
+		same_owner = __mutex_owner(lock) == owner;
+		if (same_owner) {
+			ret = owner->on_cpu;
+			if (ret)
+				cpu = task_cpu(owner);
+		}
+		rcu_read_unlock();
+
+		if (!ret || !same_owner)
+			break;
 
 		/*
 		 * Use vcpu_is_preempted to detect lock holder preemption issue.
 		 */
-		if (!owner->on_cpu || need_resched() ||
-				vcpu_is_preempted(task_cpu(owner))) {
+		if (need_resched() || vcpu_is_preempted(cpu)) {
 			ret = false;
 			break;
 		}
@@ -578,7 +588,6 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 
 		cpu_relax();
 	}
-	rcu_read_unlock();
 
 	return ret;
 }
-- 
2.28.0