linux-kernel - [PATCH v2 2/2] lib/percpu_counter: fix dying cpu compare race

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Date:   Thu,  6 Apr 2023 09:56:29 +0800
From:   Ye Bin <yebin@...weicloud.com>
To:     dennis@...nel.org, tj@...nel.org, cl@...ux.com, linux-mm@...ck.org,
        yury.norov@...il.com, andriy.shevchenko@...ux.intel.com,
        linux@...musvillemoes.dk
Cc:     linux-kernel@...r.kernel.org, dchinner@...hat.com,
        yebin10@...wei.com, yebin@...weicloud.com
Subject: [PATCH v2 2/2] lib/percpu_counter: fix dying cpu compare race

From: Ye Bin <yebin10@...wei.com>

In commit 8b57b11cca88 ("pcpcntrs: fix dying cpu summation race") a race
condition between a cpu dying and percpu_counter_sum() iterating online CPUs
was identified.
Acctually, there's the same race condition between a cpu dying and
__percpu_counter_compare(). Here, use 'num_online_cpus()' for quick judgment.
But 'num_online_cpus()' will be decreased before call 'percpu_counter_cpu_dead()',
then maybe return incorrect result.
To solve above issue, also need to add dying CPUs count when do quick judgment
in __percpu_counter_compare().

Signed-off-by: Ye Bin <yebin10@...wei.com>
---
 lib/percpu_counter.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index 5004463c4f9f..399840cb0012 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -227,6 +227,15 @@ static int percpu_counter_cpu_dead(unsigned int cpu)
 	return 0;
 }
 
+static __always_inline unsigned int num_count_cpus(void)
+{
+#ifdef CONFIG_HOTPLUG_CPU
+	return (num_online_cpus() + num_dying_cpus());
+#else
+	return num_online_cpus();
+#endif
+}
+
 /*
  * Compare counter against given value.
  * Return 1 if greater, 0 if equal and -1 if less
@@ -237,7 +246,7 @@ int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch)
 
 	count = percpu_counter_read(fbc);
 	/* Check to see if rough count will be sufficient for comparison */
-	if (abs(count - rhs) > (batch * num_online_cpus())) {
+	if (abs(count - rhs) > (batch * num_count_cpus())) {
 		if (count > rhs)
 			return 1;
 		else
-- 
2.31.1