Re: [PATCH] trace/pid_list: optimize pid_list->lock contention

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-11-10 18:38:54 [-0500], Steven Rostedt wrote:
> On Wed, 15 Oct 2025 19:49:52 +0800
> Yongliang Gao <leonylgao@gmail.com> wrote:
> 
> > diff --git a/kernel/trace/pid_list.c b/kernel/trace/pid_list.c
> > index 090bb5ea4a19..62082a4f60db 100644
> > --- a/kernel/trace/pid_list.c
> > +++ b/kernel/trace/pid_list.c
> > @@ -138,14 +138,14 @@ bool trace_pid_list_is_set(struct trace_pid_list *pid_list, unsigned int pid)
> >  	if (pid_split(pid, &upper1, &upper2, &lower) < 0)
> >  		return false;
> >  
> > -	raw_spin_lock_irqsave(&pid_list->lock, flags);
> > +	read_lock_irqsave(&pid_list->lock, flags);
> >  	upper_chunk = pid_list->upper[upper1];
> >  	if (upper_chunk) {
> >  		lower_chunk = upper_chunk->data[upper2];
> >  		if (lower_chunk)
> >  			ret = test_bit(lower, lower_chunk->data);
> >  	}
> > -	raw_spin_unlock_irqrestore(&pid_list->lock, flags);
> > +	read_unlock_irqrestore(&pid_list->lock, flags);
> >  
> >  	return ret;
> >  }
> 
> Unfortunately you cannot do this because this is called while holding the
> task pi_lock and rq locks. In PREEMPT_RT() the read/write_lock_* turn into
> mutexes.
> 
> Sebastian, is there any equivalent of raw_read/write_locks() that can be
> used?

Nope, no read-write lock that can be used in atomic sections. Well,
there is RCU.

> -- Steve

Sebastian