linux-kernel - Re: [PATCH v16 4/4] perf tools: Merge deferred user callchains

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Message-ID: <f543231e-a71c-4600-9cf3-f999ca104d86@linux.ibm.com>
Date: Wed, 12 Nov 2025 11:05:59 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: Namhyung Kim <namhyung@...nel.org>, Steven Rostedt <rostedt@...dmis.org>
Cc: Steven Rostedt <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Andrii Nakryiko <andrii@...nel.org>,
        Indu Bhagat <indu.bhagat@...cle.com>,
        "Jose E. Marchesi" <jemarch@....org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>,
        Kees Cook <kees@...nel.org>, "Carlos O'Donell" <codonell@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v16 4/4] perf tools: Merge deferred user callchains

Hello Namhyung,

could you please adapt your patches from this series to Peter's latest
changes to unwind user and related perf support, especially his new
version c69993ecdd4d ("perf: Support deferred user unwind") available
at:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git perf/core

On 10/24/2025 3:02 PM, Peter Zijlstra wrote:
> On Thu, Oct 02, 2025 at 01:49:38PM -0400, Steven Rostedt wrote:
>> On Mon, 08 Sep 2025 13:53:23 -0400
>> Steven Rostedt <rostedt@...nel.org> wrote:
>>
>>> +static int evlist__deliver_deferred_samples(struct evlist *evlist,
>>> +					    const struct perf_tool *tool,
>>> +					    union  perf_event *event,
>>> +					    struct perf_sample *sample,
>>> +					    struct machine *machine)
>>> +{
>>> +	struct deferred_event *de, *tmp;
>>> +	struct evsel *evsel;
>>> +	int ret = 0;
>>> +
>>> +	if (!tool->merge_deferred_callchains) {
>>> +		evsel = evlist__id2evsel(evlist, sample->id);
>>> +		return tool->callchain_deferred(tool, event, sample,
>>> +						evsel, machine);
>>> +	}
>>> +
>>> +	list_for_each_entry_safe(de, tmp, &evlist->deferred_samples, list) {
>>> +		struct perf_sample orig_sample;
>>
>> orig_sample is not initialized and can then contain junk.
>>
>>> +
>>> +		ret = evlist__parse_sample(evlist, de->event, &orig_sample);
>>> +		if (ret < 0) {
>>> +			pr_err("failed to parse original sample\n");
>>> +			break;
>>> +		}
>>> +
>>> +		if (sample->tid != orig_sample.tid)
>>> +			continue;
>>> +
>>> +		if (event->callchain_deferred.cookie == orig_sample.deferred_cookie)
>>> +			sample__merge_deferred_callchain(&orig_sample, sample);
>>
>> The sample__merge_deferred_callchain() initializes both
>> orig_sample.deferred_callchain and the callchain. But now that it's not
>> being called, it can cause the below free to happen with junk as the
>> callchain. This needs:
>>
>> 		else
>> 			orig_sample.deferred_callchain = false;
> 
> Ah, so I saw crashes from here and just deleted both free()s and got on
> with things ;-)

This needs to be properly resolved.  In the meantime I am using Steven's
suggestion above to continue my work on unwind user sframe (s390).

> 
>>> +
>>> +		evsel = evlist__id2evsel(evlist, orig_sample.id);
>>> +		ret = evlist__deliver_sample(evlist, tool, de->event,
>>> +					     &orig_sample, evsel,> machine); +
>>> +		if (orig_sample.deferred_callchain)
>>> +			free(orig_sample.callchain);
>>> +
>>> +		list_del(&de->list);
>>> +		free(de);
>>> +
>>> +		if (ret)
>>> +			break;
>>> +	}
>>> +	return ret;
>>> +}
>>
>> -- Steve

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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