Re: [PATCH] mm/kmemleak: report RCU-tasks quiescent states during the scan

[Paul E. McKenney]

On Thu, Jul 30, 2026 at 07:06:44AM -0700, Breno Leitao wrote:
> On Mon, Jul 27, 2026 at 06:04:37AM -0700, Breno Leitao wrote:
> > I found 3 different cases on Meta fleet, where rcu task stalls show up:
> ...
> > 3) Nvidia driver
> >   * stuck in nv_procfs_read_lock_params
> 
> I've investigated this issue. The nvidia driver contains the following
> locking pattern:
> 
>         static inline int nv_down_read_interruptible(struct rw_semaphore *lock)
>         {
>                 while (!down_read_trylock(lock))
>                 {
>                         if (signal_pending(current))
>                         return -EINTR;
>                         cond_resched();
>                 }
>                 return 0;
>         }
> 
> Code at: https://github.com/NVIDIA/open-gpu-kernel-modules/blob/452cec62d827034798072827d3866d1881662b77/kernel-open/common/inc/nv-lock.h#L54
> 
> The lock is never acquired, and cond_resched() provides insufficient
> scheduling relief.
> 
> This triggers a more severe issue with ftrace, which holds ftrace_lock
> across RCU synchronization:
> 
>         mutex_lock(&ftrace_lock);
>                 ftrace_shutdown(struct ftrace_ops *ops, int command) {
>                         synchronize_rcu_tasks_rude();
>                         synchronize_rcu_tasks();
>                 }
>         mutex_unlock(&ftrace_lock);
> 
> Since nv_down_read_interruptible() spins indefinitely, ftrace_lock
> now becomes completely wedged.
> 
> I suppose we want two things:
> 
> 1) change cond_resched() with cond_resched_tasks_rcu_qs()
> 2) Avoiding holding the ftrace lock while calling
>    synchronize_rcu_tasks()? It can take up to 10 minutes on a healthy
>    system to be releasd, right?

I do very much like both of those options.  ;-)

If #2 is impossible, would it make sense to have a mutex_lock_idle() or
similar in order to sanctify the lock synchronize_rcu_tasks() latencies?

							Thanx, Paul