Re: [PATCH v5 11/17] perf: Simplify get_perf_callchain() user logic

[Mathieu Desnoyers]

On 2025-04-24 13:28, Steven Rostedt wrote:
> On Thu, 24 Apr 2025 18:36:07 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > > +++ b/kernel/events/callchain.c
> > > @@ -246,22 +246,20 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
> > >   
> > >   	if (user) {
> > >   		if (!user_mode(regs)) {
> > > -			if  (current->mm)
> > > -				regs = task_pt_regs(current);
> > > -			else
> > > -				regs = NULL;
> > > +			if (!current->mm)
> > > +				goto exit_put;
> > > +			regs = task_pt_regs(current);
> >
> > I'm thinking this might be one of those is-kthread test written as
> > has-mm, and they're broken.
> >
> > Notably things like the io-uring kthreads do have mm.

Can a kthread such as io-uring have user_mode(regs) == true ?

> Would there ever be a case where:
>
> 	current->mm == NULL && !(current->flags & PF_KTHREAD)
>
> ?
>
> That is, do we still need to check for current->mm if it's not a kernel
> thread, or can we assume it exists?

The case I'd be careful about is if the code can nest over exit_mm()
(e.g. interrupt) after it sets current->mm = NULL.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com