Re: [PATCH v2 1/2] sched: proxy-exec: Close race causing workqueue work being delayed

[K Prateek Nayak]

Hello John,

Mostly cosmetic nitpicks. The overall idea looks good.

On 5/1/2026 3:20 AM, John Stultz wrote:
> @@ -2183,18 +2183,56 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock) __must_hold(lock);
>  #ifndef CONFIG_PREEMPT_RT
>  
>  /*
> - * With proxy exec, if a task has been proxy-migrated, it may be a donor
> - * on a cpu that it can't actually run on. Thus we need a special state
> - * to denote that the task is being woken, but that it needs to be
> - * evaluated for return-migration before it is run. So if the task is
> - * blocked_on PROXY_WAKING, return migrate it before running it.
> + * The proxy exec blocked_on pointer value uses the low bit as a latch
> + * value which clarifies if the blocked_on value is used for proxying or
> + * not.
> + *
> + * The state machine looks something like
> + *   NULL -> ptr:unlatched -> ptr:latched -> PROXY_WAKING -> NULL
> + *
> + * With some additional transitions:
> + *   ptr:unlatched -> NULL (done on current, or via set_task_blocked_on_waking())
> + *   ptr:latched -> NULL (done only on current)
> + *
> + * 1) NULL and ptr:unlatched are effectively equivalent, no proxying will occur
> + * 2) ptr:latched is the state when proxying will occur
> + * 3) PROXY_WAKING is used when the task is being woken to  ensure we
> + *    return-migrate proxy-migrated tasks before running them (note it has
> + *    the latch bit set).
>   */
> -#define PROXY_WAKING ((struct mutex *)(-1L))
> +#define PROXY_BLOCKED_LATCH (1UL)
> +#define PROXY_BLOCKED_ON_MASK(x) ((struct mutex *)((unsigned long)(x) & ~PROXY_BLOCKED_LATCH))

nit. I think PROXY_BLOCKED_ON_MUTEX() would be a better name since this
is returning the true mutex pointer back. No strong feelings, I'll defer
to others for more comments.


> +#define PROXY_WAKING ((struct mutex *)(-1L)) /* PROXY_WAKING has LATCH bit set */
> +
> +static inline struct mutex *task_is_blocked_on(struct task_struct *p)

I think this can take the role of task_is_blocked() no?

Only one caller for try_to_block_task() will require looking at the
raw blocked_on state but other than that, it is safe for the scheduler
to move around the preempted task until it has grabbed the BO latch.

> +{
> +	if (!sched_proxy_exec())
> +		return false;
> +	return (struct mutex *)((unsigned long)p->blocked_on & PROXY_BLOCKED_LATCH);
> +}
> +
> +static inline void __set_task_blocked_on_latched(struct task_struct *p)
> +{

Are you planning to reuse this sometime later in the series? If not I
think we can convert this to "try_set_task_blocked_on_latch()" and return
false if it finds blocked on having been cleared already.

That way the lock + check in try_to_block_task() can be moved here.

> +	lockdep_assert_held_once(&p->blocked_lock);
> +	WARN_ON_ONCE(!p->blocked_on);
> +	p->blocked_on = (struct mutex *)((unsigned long)p->blocked_on | PROXY_BLOCKED_LATCH);
> +}
> +
> +static inline struct mutex *__get_task_latched_blocked_on(struct task_struct *p)

I think this can be __get_task_blocked_on() ...

> +{
> +	if (!task_is_blocked_on(p))
> +		return NULL;
> +	if (p->blocked_on == PROXY_WAKING)
> +		return PROXY_WAKING;
> +	return PROXY_BLOCKED_ON_MASK(p->blocked_on);
> +}
>  
>  static inline struct mutex *__get_task_blocked_on(struct task_struct *p)

... and this can be __get_task_blocked_on_raw() since only one caller in
kernel/locking/mutex.h really cares about the ~PROXY_BLOCKED_LATCH
value outside of this file.

Everything in the sched bits can then simply be __get_task_blocked_on()
and that seems much cleaner.

Thoughts?

>  {
>  	lockdep_assert_held_once(&p->blocked_lock);
> -	return p->blocked_on == PROXY_WAKING ? NULL : p->blocked_on;
> +	if (p->blocked_on == PROXY_WAKING)
> +		return NULL;
> +	return PROXY_BLOCKED_ON_MASK(p->blocked_on);
>  }
>  
>  static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
> @@ -2215,6 +2253,8 @@ static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
>  
>  static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m)
>  {
> +	struct mutex *bo = p->blocked_on;
> +
>  	/* Currently we serialize blocked_on under the task::blocked_lock */
>  	lockdep_assert_held_once(&p->blocked_lock);
>  	/*
> @@ -2222,7 +2262,7 @@ static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *
>  	 * 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 != PROXY_WAKING);
> +	WARN_ON_ONCE(m && bo && __get_task_blocked_on(p) != m && bo != PROXY_WAKING);
>  	p->blocked_on = NULL;
>  }
>  
> @@ -2242,15 +2282,17 @@ static inline void __set_task_blocked_on_waking(struct task_struct *p, struct mu
>  		return;
>  	}
>  
> -	/* Don't set PROXY_WAKING if blocked_on was already cleared */
> -	if (!p->blocked_on)
> +	/* Don't set PROXY_WAKING if we are not really blocked_on  */
> +	if (!task_is_blocked_on(p)) {
> +		p->blocked_on = NULL;  /* clear if unlatched */
>  		return;
> +	}
>  	/*
>  	 * There may be cases where we set PROXY_WAKING on tasks that were
>  	 * already set to waking, but make sure we are not changing
>  	 * the relationship with a different lock.
>  	 */
> -	WARN_ON_ONCE(m && p->blocked_on != m && p->blocked_on != PROXY_WAKING);
> +	WARN_ON_ONCE(m && __get_task_blocked_on(p) != m && p->blocked_on != PROXY_WAKING);
>  	p->blocked_on = PROXY_WAKING;
>  }
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index da20fb6ea25ae..2f912bf698446 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6599,8 +6599,13 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
>  	 * blocked on a mutex, and we want to keep it on the runqueue
>  	 * to be selectable for proxy-execution.
>  	 */
> -	if (!should_block)
> -		return false;
> +	if (!should_block) {
> +		guard(raw_spinlock)(&p->blocked_lock);
> +		if (p->blocked_on) {
> +			__set_task_blocked_on_latched(p);
> +			return false;
> +		}
> +	}

In my head, this as:

    if (!should_block & try_to_latch_task_blocked_on(p))
           return false;

seems much cleaner. I'll defer to other for comments.

>  
>  	p->sched_contributes_to_load =
>  		(task_state & TASK_UNINTERRUPTIBLE) &&
> @@ -6833,7 +6838,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  	int owner_cpu;
>  
>  	/* Follow blocked_on chain. */
> -	for (p = donor; (mutex = p->blocked_on); p = owner) {
> +	for (p = donor; (mutex = __get_task_latched_blocked_on(p)); p = owner) {
>  		/* if its PROXY_WAKING, do return migration or run if current */
>  		if (mutex == PROXY_WAKING) {
>  			if (task_current(rq, p)) {
> @@ -6851,7 +6856,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  		guard(raw_spinlock)(&p->blocked_lock);
>  
>  		/* Check again that p is blocked with blocked_lock held */
> -		if (mutex != __get_task_blocked_on(p)) {
> +		if (mutex != __get_task_latched_blocked_on(p)) {
>  			/*
>  			 * Something changed in the blocked_on chain and
>  			 * we don't know if only at this level. So, let's
> @@ -7107,7 +7112,7 @@ static void __sched notrace __schedule(int sched_mode)
>  		struct task_struct *prev_donor = rq->donor;
>  
>  		rq_set_donor(rq, next);
> -		if (unlikely(next->blocked_on)) {
> +		if (unlikely(task_is_blocked_on(next))) {
>  			next = find_proxy_task(rq, next, &rf);
>  			if (!next) {
>  				zap_balance_callbacks(rq);

-- 
Thanks and Regards,
Prateek