Re: [PATCH v22 1/6] locking: Add task::blocked_lock to serialize blocked_on state

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 26, 2025 at 03:29:09AM +0000, John Stultz wrote:
> So far, we have been able to utilize the mutex::wait_lock
> for serializing the blocked_on state, but when we move to
> proxying across runqueues, we will need to add more state
> and a way to serialize changes to this state in contexts
> where we don't hold the mutex::wait_lock.
> 
> So introduce the task::blocked_lock, which nests under the
> mutex::wait_lock in the locking order, and rework the locking
> to use it.
> 
> Signed-off-by: John Stultz <jstultz@google.com>
> Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>  include/linux/sched.h        | 52 +++++++++++++++---------------------
>  init/init_task.c             |  1 +
>  kernel/fork.c                |  1 +
>  kernel/locking/mutex-debug.c |  4 +--
>  kernel/locking/mutex.c       | 40 +++++++++++++++++----------
>  kernel/locking/ww_mutex.h    |  4 +--
>  kernel/sched/core.c          |  4 ++-
>  7 files changed, 57 insertions(+), 49 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index e4ce0a76831e5..cb4e81d9d9b67 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h

> +static inline struct mutex *get_task_blocked_on(struct task_struct *p)
> +{
> +	guard(raw_spinlock_irqsave)(&p->blocked_lock);
> +	return __get_task_blocked_on(p);
>  }

This isn't a safe function in general; nothing guarantees the value
returned is stable. Perhaps move it inside kernel/locking/mutex.h, its
only users (below) are mutex debug code after all.

> diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
> index 949103fd8e9b5..1d8cff71f65e1 100644
> --- a/kernel/locking/mutex-debug.c
> +++ b/kernel/locking/mutex-debug.c
> @@ -54,13 +54,13 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
>  	lockdep_assert_held(&lock->wait_lock);
>  
>  	/* Current thread can't be already blocked (since it's executing!) */
> -	DEBUG_LOCKS_WARN_ON(__get_task_blocked_on(task));
> +	DEBUG_LOCKS_WARN_ON(get_task_blocked_on(task));
>  }
>  
>  void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
>  			 struct task_struct *task)
>  {
> -	struct mutex *blocked_on = __get_task_blocked_on(task);
> +	struct mutex *blocked_on = get_task_blocked_on(task);
>  
>  	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
>  	DEBUG_LOCKS_WARN_ON(waiter->task != task);
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index de7d6702cd96c..c44fc63d4476e 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c

> @@ -740,11 +752,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
>  	return 0;
>  
>  err:
> -	__clear_task_blocked_on(current, lock);
> +	clear_task_blocked_on(current, lock);
>  	__set_current_state(TASK_RUNNING);
>  	__mutex_remove_waiter(lock, &waiter);
>  err_early_kill:
> -	WARN_ON(__get_task_blocked_on(current));
> +	WARN_ON(get_task_blocked_on(current));
>  	trace_contention_end(lock, ret);
>  	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
>  	debug_mutex_free_waiter(&waiter);