Re: [RESEND][PATCH v18 3/8] locking/mutex: Add p->blocked_on wrappers for correctness checks

[K Prateek Nayak]

Hello John,

On 7/8/2025 2:13 AM, John Stultz wrote:
> @@ -2177,6 +2178,57 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
>  	__cond_resched_rwlock_write(lock);					\
>  })
>  
> +static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	WARN_ON_ONCE(!m);
> +	/* The task should only be setting itself as blocked */
> +	WARN_ON_ONCE(p != current);
> +	/* Currently we serialize blocked_on under the mutex::wait_lock */
> +	lockdep_assert_held_once(&m->wait_lock);

Sorry I didn't catch this earlier but building this with PREEMPT_RT fails
with the following error:

    ./include/linux/sched.h: In function ‘__set_task_blocked_on’:
    ./include/linux/sched.h:2187:36: error: ‘struct mutex’ has no member named ‘wait_lock’
     2187 |         lockdep_assert_held_once(&m->wait_lock);


Putting all these helpers behind a "#ifdef CONFIG_PREEMPT_RT" will then
lead to the this error:

    kernel/locking/ww_mutex.h:292:17: error: implicit declaration of function ‘__clear_task_blocked_on’ [-Werror=implicit-function-declaration]
      292 |                 __clear_task_blocked_on(waiter->task, lock);


Putting the "__clear_task_blocked_on()" calls in kernel/locking/ww_mutex.h
behind "#ifndef WW_RT" (which should start from Patch 2 since MUTEX and
MUTEX_WAITER for ww_mutex will resolve to rt_mutex and rt_mutex_waiter in
presence of WW_RT) solves all build issues with PREEMPT_RT. I'll let others
comment on the correctness tho :)


Following is the diff I used on top of this series for reference:

(build and boot tested only)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1d7f625adbb5..d47c9e4edf4f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2178,6 +2178,8 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
 	__cond_resched_rwlock_write(lock);					\
 })
 
+#ifndef CONFIG_PREEMPT_RT
+
 static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
 {
 	WARN_ON_ONCE(!m);
@@ -2229,6 +2231,8 @@ static inline struct mutex *__get_task_blocked_on(struct task_struct *p)
 	return m;
 }
 
+#endif
+
 static __always_inline bool need_resched(void)
 {
 	return unlikely(tif_need_resched());
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 086fd5487ca7..fd67a4a49892 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -283,13 +283,14 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
 	if (waiter->ww_ctx->acquired > 0 && __ww_ctx_less(waiter->ww_ctx, ww_ctx)) {
 #ifndef WW_RT
 		debug_mutex_wake_waiter(lock, waiter);
-#endif
+
 		/*
 		 * When waking up the task to die, be sure to clear the
 		 * blocked_on pointer. Otherwise we can see circular
 		 * blocked_on relationships that can't resolve.
 		 */
 		__clear_task_blocked_on(waiter->task, lock);
+#endif
 		wake_q_add(wake_q, waiter->task);
 	}
 
@@ -338,12 +339,14 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
 		 * wakeup pending to re-read the wounded state.
 		 */
 		if (owner != current) {
+#ifndef WW_RT
 			/*
 			 * When waking up the task to wound, be sure to clear the
 			 * blocked_on pointer. Otherwise we can see circular
 			 * blocked_on relationships that can't resolve.
 			 */
 			__clear_task_blocked_on(owner, lock);
+#endif
 			wake_q_add(wake_q, owner);
 		}
 		return true;
---

I'll make sure to give the PREEMPT_RT build a spin next time around.
Sorry for the oversight.

> +	/*
> +	 * Check ensure we don't overwrite existing mutex value
> +	 * with a different mutex. Note, setting it to the same
> +	 * lock repeatedly is ok.
> +	 */
> +	WARN_ON_ONCE(p->blocked_on && p->blocked_on != m);
> +	p->blocked_on = m;
> +}
> +
> +static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	guard(raw_spinlock_irqsave)(&m->wait_lock);
> +	__set_task_blocked_on(p, m);
> +}
> +
> +static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	WARN_ON_ONCE(!m);
> +	/* Currently we serialize blocked_on under the mutex::wait_lock */
> +	lockdep_assert_held_once(&m->wait_lock);
> +	/*
> +	 * There may be cases where we re-clear already cleared
> +	 * blocked_on relationships, but make sure we are not
> +	 * clearing the relationship with a different lock.
> +	 */
> +	WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m);
> +	p->blocked_on = NULL;
> +}
> +
> +static inline void clear_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	guard(raw_spinlock_irqsave)(&m->wait_lock);
> +	__clear_task_blocked_on(p, m);
> +}
> +
> +static inline struct mutex *__get_task_blocked_on(struct task_struct *p)
> +{
> +	struct mutex *m = p->blocked_on;
> +
> +	if (m)
> +		lockdep_assert_held_once(&m->wait_lock);
> +	return m;
> +}
> +
>  static __always_inline bool need_resched(void)
>  {
>  	return unlikely(tif_need_resched());


-- 
Thanks and Regards,
Prateek