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Date: Fri, 25 Apr 2025 09:13:39 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Josh Poimboeuf <jpoimboe@kernel.org>, x86@kernel.org,
	Ingo Molnar <mingo@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Indu Bhagat <indu.bhagat@oracle.com>,
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	Adrian Hunter <adrian.hunter@intel.com>,
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	Beau Belgrave <beaub@linux.microsoft.com>,
	"Jose E. Marchesi" <jemarch@gnu.org>
Subject: Re: [PATCH v5 11/17] perf: Simplify get_perf_callchain() user logic
Message-ID: <20250425071339.GJ18306@noisy.programming.kicks-ass.net>
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In-Reply-To: <20250424132800.49160c2e@gandalf.local.home>

On Thu, Apr 24, 2025 at 01:28:00PM -0400, Steven Rostedt wrote:
> On Thu, 24 Apr 2025 18:36:07 +0200
> Peter Zijlstra <peterz@infradead.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?

IIRC just checking PF_KTHREAD should be sufficient.