Re: [PATCH v13 10/14] unwind: Clear unwind_mask on exit back to user space

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, Jul 15, 2025 at 01:20:16PM -0400, Steven Rostedt wrote:
> On Tue, 15 Jul 2025 12:29:12 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > @@ -170,41 +193,62 @@ static void unwind_deferred_task_work(st
> >  int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
> >  {
> >  	struct unwind_task_info *info = &current->unwind_info;
> > -	int ret;
> > +	unsigned long bits, mask;
> > +	int bit, ret;
> >  
> >  	*cookie = 0;
> >  
> > -	if (WARN_ON_ONCE(in_nmi()))
> > -		return -EINVAL;
> > -
> >  	if ((current->flags & (PF_KTHREAD | PF_EXITING)) ||
> >  	    !user_mode(task_pt_regs(current)))
> >  		return -EINVAL;
> >  
> > +	/* NMI requires having safe cmpxchg operations */
> > +	if (WARN_ON_ONCE(!UNWIND_NMI_SAFE && in_nmi()))
> > +		return -EINVAL;
> 
> I don't think we want to have a WARN_ON() here as the perf series tries
> to first do the deferred unwinding and if that fails, it will go back
> to it's old method.

The thing is, I don't think we have an architecture that supports NMIs
and does not have NMI safe cmpxchg. And if we do have one such -- I
don't think it has perf; perf very much assumes cmpxchg is NMI safe.

Calling this from NMI context and not having an NMI safe cmpxchg is very
much a dodgy use case. Please leave the WARN, if it ever triggers, we'll
look at who manages and deal with it then.