Re: [PATCH] sched: Fix cpuset_mutex double unlock in __sched_setscheduler()

[Juri Lelli <juri.lelli@redhat.com>]

Hello,

On 02/11/26 14:46, Andrea Parri wrote:
> A sched_setattr() with SCHED_FLAG_KEEP_POLICY on a SCHED_DEADLINE task
> unlocks cpuset_mutex twice if the task's policy changes concurrently:
> 
>   WARNING: bad unlock balance detected!
>   7.4.0-rc1 #1 Not tainted
>   -------------------------------------
>   repro/126 is trying to release lock (cpuset_mutex) at:
>   [<ffffffff9a17cacc>] __sched_setscheduler+0x9cc/0xc40
>   but there are no more locks to release!
>   ...
>   Call Trace:
>    print_unlock_imbalance_bug.part.0+0xfb/0x110
>    lock_release+0x286/0x2e0
>    __mutex_unlock_slowpath+0x1a/0x400
>    __sched_setscheduler+0x9cc/0xc40
>    __x64_sys_sched_setattr+0xa3/0x1b0
>    do_syscall_64+0x106/0x5f0
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> __sched_setscheduler() samples p->policy before taking any lock, and
> restarts from the recheck label if p->policy differs under the rq lock.
> The restart drops cpuset_mutex but leaves cpuset_locked set, so a pass
> that no longer needs the mutex still unlocks it on its way out:
> 
>   CPU 1                                CPU 2
>   __sched_setscheduler(KEEP_POLICY)
>     policy = p->policy /* DEADLINE */
>                                        __sched_setscheduler(SCHED_OTHER)
>                                          p->policy = SCHED_OTHER
>     cpuset_locked = true
>     cpuset_lock()
>     rq = task_rq_lock(p)
>     oldpolicy != p->policy
>       cpuset_unlock()
>       goto recheck
>     /* no cpuset_lock() this time */
>     ...
>     if (cpuset_locked)
>       cpuset_unlock()
> 
> Without CONFIG_DEBUG_MUTEXES, or CONFIG_DEBUG_RT_MUTEXES on PREEMPT_RT,
> nothing warns. The second unlock still clears the owner field regardless
> of who holds the mutex, so another task can lose cpuset_mutex inside its
> critical section. Entering SCHED_DEADLINE needs CAP_SYS_NICE, but a
> racing caller that owns the task does not need it if it samples a policy
> other than SCHED_DEADLINE.
> 
> Clear cpuset_locked when dropping the mutex for the restart. This should be
> safe because the flag is only tested on the paths leaving the next pass,
> after that pass has set it again if it took the mutex.
> 
> The following program, run as root, reproduces the splat, which no longer
> appears with this applied. On PREEMPT_RT it crashes the kernel within
> seconds, which this also fixes:
> 
>   #define _GNU_SOURCE
>   #include <pthread.h>
>   #include <stdint.h>
>   #include <sys/syscall.h>
>   #include <unistd.h>
> 
>   struct attr {
>       uint32_t size, policy;
>       uint64_t flags;
>       int32_t nice;
>       uint32_t prio;
>       uint64_t runtime, deadline, period;
>   };
> 
>   static volatile int stop;
>   static volatile pid_t target;
> 
>   static void set(struct attr a)
>   {
>       a.size = sizeof(a);
>       syscall(SYS_sched_setattr, target, &a, 0);
>   }
> 
>   static void *sleeper(void *arg)
>   {
>       target = syscall(SYS_gettid);
>       while (!stop)
>           usleep(1000);
>       return arg;
>   }
> 
>   static void *switcher(void *arg)
>   {
>       while (!stop) {
>           /* SCHED_DEADLINE, 2us every 1s */
>           set((struct attr){ .policy = 6, .runtime = 2000,
>                              .deadline = 1000000000,
>                              .period = 1000000000 });
>           set((struct attr){ .policy = 0 }); /* SCHED_OTHER */
>       }
>       return arg;
>   }
> 
>   static void *racer(void *arg)
>   {
>       /* KEEP_POLICY | KEEP_PARAMS */
>       while (!stop)
>           set((struct attr){ .flags = 0x18 });
>       return arg;
>   }
> 
>   int main(void)
>   {
>       pthread_t t[5];
>       int i;
> 
>       pthread_create(&t[0], NULL, sleeper, NULL);
>       while (!target)
>           usleep(100);
>       pthread_create(&t[1], NULL, switcher, NULL);
>       for (i = 2; i < 5; i++)
>           pthread_create(&t[i], NULL, racer, NULL);
>       sleep(30);
>       stop = 1;
>       for (i = 0; i < 5; i++)
>           pthread_join(t[i], NULL);
>       return 0;
>   }
> 
> An LLM helped find the PREEMPT_RT crash and write this patch and its test.
> 
> Fixes: 111cd11bbc54 ("sched/cpuset: Bring back cpuset_mutex")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Link: https://sashiko.dev/#/patchset/20261016062629.2102883-1-sh_def%40163.com
> Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
> Cc: stable@vger.kernel.org
> ---

The fix looks good to me.

Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

As I said in the other fix, I'm not sure we want to keep the reproducer
in the changelog, or if it's better to start collecting them somewhere
else (it might be easier to actually run them).

Thanks,
Juri