Re: [PATCH v2] coresight: Fix scheduling while atomic in coresight_put_percpu_source_ref()

[Sebastian Andrzej Siewior]

On 2026-07-14 02:00:27 [+0300], Mohamed Ayman wrote:
> Dropping the last reference to a coresight_device triggers a kernel panic
> on PREEMPT_RT builds due to a "scheduling while atomic" violation.
> 
> During CPU idle transitions, coresight_cpu_pm_notify() runs with
> interrupts disabled. It eventually calls put_device(), which can
> synchronously trigger the device's release callback and drop the parent
> device's reference. On PREEMPT_RT, free_percpu() takes a sleeping lock
> (rt-mutex), and the parent's release callback might also sleep. Sleeping

It is a spinlock_t which we refer as a sleeping lock. There is "struct
rt_mutex" which is somehow different.
I would suggest to word it like "uses a spinlock_t for locking which
becomes a sleeping lock on PREEMPT_RT".

> in this atomic PM context crashes the system.
> 
> A previous patch tried deferring just the coresight_device_release() body,
> but this still left the synchronous put_device() call dangerously exposed
> to sleeping parent release functions.
> 
> Fix this by entirely deferring the put_device() call to process context.
> We add a pending counter (put_pending) and a work_struct to the coresight
> device. When releasing a reference, we increment the counter and queue
> the work. A worker thread then safely drains the counter and calls
> put_device(). The counter prevents leaking references if multiple puts
> are queued before the worker even has a chance to run.
> 
> To prevent a use-after-free race condition during module unload, the work
> is queued on a dedicated coresight_wq which is safely drained and
> destroyed in coresight_exit().

Do you have anything that keeps the module-ref counter up with each new
device?

> Finally, remove the unnecessary raw_spinlock_irqsave in the put path,
> as dropping a reference doesn't require protecting the per-CPU table.
> 
> Signed-off-by: Mohamed Ayman <mohamedaymanworkspace@xxxxxxxxx>
> ---
…
> @@ -163,16 +175,9 @@ void coresight_put_percpu_source_ref(struct coresight_device *csdev)
>  	if (!csdev || !coresight_is_percpu_source(csdev))
>  		return;
>  
> -	guard(raw_spinlock_irqsave)(&coresight_dev_lock);
> +	atomic_inc(&csdev->put_pending);
>  
> -	/*
> -	 * TODO: coresight_device_release() is invoked to release resources when
> -	 * the device's refcount reaches zero. It then calls free_percpu(),
> -	 * which acquires pcpu_lock — a sleepable lock when PREEMPT_RT is
> -	 * enabled. Since the raw spinlock coresight_dev_lock is held, this can
> -	 * lead to a potential "scheduling while atomic" issue.
> -	 */
> -	put_device(&csdev->dev);
> +	queue_work(coresight_wq, &csdev->put_work);

What about you keep this as-is and just delay coresight_device_release()
instead?

>  }
>  
>  struct coresight_device *coresight_get_source(struct coresight_path *path)

Sebastian