Re: [PATCH v3 1/2] futex: Add missing rt_mutex_.*_schedule() around rt_mutex_wait_proxy_lock()

[Peter Zijlstra]

On Mon, Aug 24, 2026 at 02:55:42PM +0200, Sebastian Andrzej Siewior wrote:
> From: Yao Kai <yaokai34@xxxxxxxxxx>
> 
> A waiter requeued onto a PI futex can reach rt_mutex_wait_proxy_lock()
> without rtmutex schedule preparation, triggering the lockdep_assert() in
> rt_mutex_schedule().
> The lack of it, can be seen with requeue PI, multiple waiters and
> requeing multiple tasks, the subsequent requeued task can be requeued in
> the state Q_REQUEUE_PI_DONE:
> 
>         waiter                          requeue task
>         ------                          ------------
> futex_wait_requeue_pi()
>   futex_wait_setup()
>     futex_queue(&q)
>                                         futex_requeue()
>                                           futex_proxy_trylock_atomic()
>                                           futex_requeue_pi_prepare()
>                                            Q_REQUEUE_PI_NONE->Q_REQUEUE_PI_IN_PROGRESS
> 
>                                           rt_mutex_start_proxy_lock() (ret = 0)
>                                           requeue_futex()
>   futex_do_wait()
>                                           futex_requeue_pi_complete()
>                                             Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_DONE
>   futex_requeue_pi_wakeup_sync()
>   rt_mutex_wait_proxy_lock()
>     rt_mutex_schedule() (on contention)
> 
> In the Q_REQUEUE_PI_DONE case the waiter will acquire the pi_mutex.
> Should the lock be contended, the waiter will invoke rt_mutex_schedule()
> without invoking rt_mutex_.*_schedule() before/ after scheduling.
> 
> Invoke rt_mutex_pre_schedule() and rt_mutex_post_schedule() directly around
> rt_mutex_wait_proxy_lock().

Still doesn't explain why this is makes sense. Perhaps something like
the below comment to demystify the code?

> [bigeasy: Redid parts of the changelog, dropped the comment misleading]
> 
> Fixes: d14f9e930b90 ("locking/rtmutex: Use rt_mutex specific scheduler helpers")
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Signed-off-by: Yao Kai <yaokai34@xxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>  kernel/futex/requeue.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
> index 79823ad136830..d8c9e7d218695 100644
> --- a/kernel/futex/requeue.c
> +++ b/kernel/futex/requeue.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0-or-later
>  
>  #include <linux/plist.h>
> +#include <linux/sched/rt.h>
>  #include <linux/sched/signal.h>
>  
>  #include "futex.h"
> @@ -865,7 +866,10 @@ int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
>  	case Q_REQUEUE_PI_DONE:
>  		/* Requeue completed. Current is 'pi_blocked_on' the rtmutex */
>  		pi_mutex = &q.pi_state->pi_mutex;
> +

		/*
		 * Since current is doing the requeue, it cannot also be
		 * a waiter on the same futex.
		 */

> +		rt_mutex_pre_schedule();
>  		ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);
> +		rt_mutex_post_schedule();
>  
>  		/*
>  		 * See futex_unlock_pi()'s cleanup: comment.
> -- 
> 2.55.0
>