linux-kernel - Re: [PATCH v5 11/17] perf: Simplify get_perf

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Message-ID: <20250424132800.49160c2e@gandalf.local.home>
Date: Thu, 24 Apr 2025 13:28:00 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
 x86@...nel.org, Ingo Molnar <mingo@...nel.org>, Arnaldo Carvalho de Melo
 <acme@...nel.org>, Indu Bhagat <indu.bhagat@...cle.com>, Alexander Shishkin
 <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian
 Hunter <adrian.hunter@...el.com>, linux-perf-users@...r.kernel.org, Mark
 Brown <broonie@...nel.org>, linux-toolchains@...r.kernel.org, Jordan Rome
 <jordalgo@...a.com>, Sam James <sam@...too.org>, Andrii Nakryiko
 <andrii.nakryiko@...il.com>, Jens Remus <jremus@...ux.ibm.com>, Florian
 Weimer <fweimer@...hat.com>, Andy Lutomirski <luto@...nel.org>, Weinan Liu
 <wnliu@...gle.com>, Blake Jones <blakejones@...gle.com>, Beau Belgrave
 <beaub@...ux.microsoft.com>, "Jose E. Marchesi" <jemarch@....org>
Subject: Re: [PATCH v5 11/17] perf: Simplify get_perf_callchain() user logic

On Thu, 24 Apr 2025 18:36:07 +0200
Peter Zijlstra <peterz@...radead.org> 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.

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?

-- Steve