linux-kernel - [PATCH] locking/qspinlock: Do spin-wait in slowpath if preemptible

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Date:   Tue, 20 Sep 2022 15:39:51 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: [PATCH] locking/qspinlock: Do spin-wait in slowpath if preemptible

There are some code paths in the kernel where arch_spin_lock() will be
called directly when the lock isn't expected to be contended and critical
section is short. For example, tracing_saved_cmdlines_size_read()
in kernel/trace/trace.c does that.

In most cases, preemption is also not disabled. This creates a problem
for the qspinlock slowpath which expects preemption to be disabled
to guarantee the safe use of per cpu qnodes structure. To work around
these special use cases, add a preemption count check in the slowpath
and do a simple spin-wait when preemption isn't disabled.

Fixes: a33fda35e3a7 ("Introduce a simple generic 4-byte queued spinlock")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/locking/qspinlock.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 65a9a10caa6f..5e352cf8fa18 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -321,6 +321,23 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 
 	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
 
+#ifdef CONFIG_PREEMPT_COUNT
+	/*
+	 * As arch_spin_lock() can be called directly in some use cases
+	 * where the lock isn't expected to be contended, critical section
+	 * is short and preemption isn't disabled, we can't use qnodes in
+	 * this case as state may be screwed up in case preemption happens
+	 * or preemption warning may be printed (CONFIG_DEBUG_PREEMPT).
+	 * Just do a simple spin-wait in this case as the lock shouldn't be
+	 * contended for long.
+	 */
+	if (unlikely(!preempt_count())) {
+		while (!queued_spin_trylock(lock))
+			cpu_relax();
+		return;
+	}
+#endif
+
 	if (pv_enabled())
 		goto pv_queue;
 
-- 
2.31.1