From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
"Paul E. McKenney" <paulmck@kernel.org>
Subject: [PATCH next v4 2/2] printk: fix cpu lock ordering
Date: Thu, 17 Jun 2021 11:56:51 +0206 [thread overview]
Message-ID: <20210617095051.4808-3-john.ogness@linutronix.de> (raw)
In-Reply-To: <20210617095051.4808-1-john.ogness@linutronix.de>
The cpu lock implementation uses a full memory barrier to take
the lock, but no memory barriers when releasing the lock. This
means that changes performed by a lock owner may not be seen by
the next lock owner. This may have been "good enough" for use
by dump_stack() as a serialization mechanism, but it is not
enough to provide proper protection for a critical section.
Correct this problem by using acquire/release memory barriers
for lock/unlock, respectively.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk.c | 53 +++++++++++++++++++++++++++++++++++++++---
1 file changed, 50 insertions(+), 3 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 08e14a67c44e..5376216e4f3d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3569,10 +3569,33 @@ int __printk_cpu_trylock(void)
cpu = smp_processor_id();
- old = atomic_cmpxchg(&printk_cpulock_owner, -1, cpu);
+ /*
+ * Guarantee loads and stores from this CPU when it is the lock owner
+ * are _not_ visible to the previous lock owner. This pairs with
+ * __printk_cpu_unlock:B.
+ *
+ * Memory barrier involvement:
+ *
+ * If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B, then
+ * __printk_cpu_unlock:A can never read from __printk_cpu_trylock:B.
+ *
+ * Relies on:
+ *
+ * RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
+ * of the previous CPU
+ * matching
+ * ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
+ * of this CPU
+ */
+ old = atomic_cmpxchg_acquire(&printk_cpulock_owner, -1,
+ cpu); /* LMM(__printk_cpu_trylock:A) */
if (old == -1) {
- /* This CPU is now the owner. */
+ /*
+ * This CPU is now the owner and begins loading/storing
+ * data: LMM(__printk_cpu_trylock:B)
+ */
return 1;
+
} else if (old == cpu) {
/* This CPU is already the owner. */
atomic_inc(&printk_cpulock_nested);
@@ -3597,7 +3620,31 @@ void __printk_cpu_unlock(void)
return;
}
- atomic_set(&printk_cpulock_owner, -1);
+ /*
+ * This CPU is finished loading/storing data:
+ * LMM(__printk_cpu_unlock:A)
+ */
+
+ /*
+ * Guarantee loads and stores from this CPU when it was the
+ * lock owner are visible to the next lock owner. This pairs
+ * with __printk_cpu_trylock:A.
+ *
+ * Memory barrier involvement:
+ *
+ * If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B,
+ * then __printk_cpu_trylock:B reads from __printk_cpu_unlock:A.
+ *
+ * Relies on:
+ *
+ * RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
+ * of this CPU
+ * matching
+ * ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
+ * of the next CPU
+ */
+ atomic_set_release(&printk_cpulock_owner,
+ -1); /* LMM(__printk_cpu_unlock:B) */
}
EXPORT_SYMBOL(__printk_cpu_unlock);
#endif /* CONFIG_SMP */
--
2.20.1
next prev parent reply other threads:[~2021-06-17 9:51 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-17 9:50 [PATCH next v4 0/2] introduce printk cpu lock John Ogness
2021-06-17 9:50 ` [PATCH next v4 1/2] lib/dump_stack: move cpu lock to printk.c John Ogness
2021-06-17 13:32 ` Steven Rostedt
2021-06-18 14:47 ` Petr Mladek
2021-06-18 16:25 ` Steven Rostedt
2021-06-19 0:22 ` John Ogness
2021-06-18 14:55 ` John Ogness
2021-06-18 16:31 ` Steven Rostedt
2021-06-17 9:50 ` John Ogness [this message]
2021-06-17 11:23 ` [PATCH next v4 0/2] introduce printk cpu lock Petr Mladek
2021-06-17 11:28 ` Stephen Rothwell
2021-06-17 11:39 ` Sergey Senozhatsky
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