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

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Message-ID: <2fe98d8a-1165-45ac-b539-d597dd1bbcaa@efficios.com>
Date: Thu, 24 Apr 2025 13:42:16 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>,
 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>,
 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 2025-04-24 13:28, Steven Rostedt wrote:
> 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.

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