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

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, Aug 24 2026 at 15:23, Peter Zijlstra wrote:
> On Mon, Aug 24, 2026 at 02:55:42PM +0200, Sebastian Andrzej Siewior wrote:
>>  #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.
> 		 */

Current is not the task doing the requeue. Current is the requeued waiter which is
blocked on the rtmutex.

Though the change log is confusing at best. The whole requeue explanation
is not really helpful IMO.

The point is that _all_ invocations of rt_mutex_schedule() must be
preceeded by a call to rt_mutex_pre_schedule() and followed by a call to
rt_mutex_post_schedule().

So this code simply failed to do that, which triggers the
lockdep_assert() in rt_mutex_schedule().

And it's obvious from the surrounding comments how this task got there, no?

>> +		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
>>