linux-kernel - [PATCH] sched: Fix race in rt_mutex_pre_schedule by removing non-atomic fetch_and

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Message-Id: <20250826110833.3274209-1-cuiguoqi@kylinos.cn>
Date: Tue, 26 Aug 2025 19:08:33 +0800
From: cuiguoqi <cuiguoqi@...inos.cn>
To: Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	guoqi0226@....com,
	linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev,
	cuiguoqi <cuiguoqi@...inos.cn>
Subject: [PATCH] sched: Fix race in rt_mutex_pre_schedule by removing non-atomic fetch_and_set

During Wound/Wait testing on PREEMPT_RT, a WARNING was hit:

  WARNING: CPU: 0 PID: 0 at kernel/sched/core.c:7085 rt_mutex_pre_schedule+0xa8/0x108
  Call trace:
   rt_mutex_pre_schedule+0xa8/0x108
   __ww_rt_mutex_lock+0x1d4/0x300
   ww_mutex_lock+0x1c/0x30

The issue stems from the non-atomic `fetch_and_set` macro:
    #define fetch_and_set(x, v) ({ int _x = (x); (x) = (v); _x; })

It lacks atomicity and memory ordering, leading to race conditions under
preemption or interrupts, where `current->sched_rt_mutex` may be corrupted.

Since this flag is only used for lockdep assertions and accessed per-task,
replace the unsafe macro with direct assignment and explicit state checks:

- In rt_mutex_pre_schedule(): assert is 0 before setting to 1.
- In rt_mutex_post_schedule(): assert is 1 before clearing to 0.

This fixes the false-positive warning without needing atomic operations.

Signed-off-by: cuiguoqi <cuiguoqi@...inos.cn>
---
 kernel/sched/core.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5e89a6eeadba..fb4c446e46f7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7078,11 +7078,11 @@ const struct sched_class *__setscheduler_class(struct task_struct *p, int prio)
  * name such that if someone were to implement this function we get to compare
  * notes.
  */
-#define fetch_and_set(x, v) ({ int _x = (x); (x) = (v); _x; })
 
 void rt_mutex_pre_schedule(void)
 {
-	lockdep_assert(!fetch_and_set(current->sched_rt_mutex, 1));
+	lockdep_assert(!current->sched_rt_mutex);
+	current->sched_rt_mutex = 1;
 	sched_submit_work(current);
 }
 
@@ -7095,7 +7095,9 @@ void rt_mutex_schedule(void)
 void rt_mutex_post_schedule(void)
 {
 	sched_update_worker(current);
-	lockdep_assert(fetch_and_set(current->sched_rt_mutex, 0));
+	lockdep_assert(current->sched_rt_mutex);
+	current->sched_rt_mutex = 0;
+
 }
 
 /*
-- 
2.25.1