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Date: Thu, 24 Apr 2025 18:38:45 +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>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
	Ian Rogers <irogers@google.com>,
	Adrian Hunter <adrian.hunter@intel.com>,
	linux-perf-users@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	linux-toolchains@vger.kernel.org, Jordan Rome <jordalgo@meta.com>,
	Sam James <sam@gentoo.org>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
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	Andy Lutomirski <luto@kernel.org>, Weinan Liu <wnliu@google.com>,
	Blake Jones <blakejones@google.com>,
	Beau Belgrave <beaub@linux.microsoft.com>,
	"Jose E. Marchesi" <jemarch@gnu.org>
Subject: Re: [PATCH v5 13/17] perf: Support deferred user callchains
Message-ID: <20250424163845.GG18306@noisy.programming.kicks-ass.net>
References: <20250424162529.686762589@goodmis.org>
 <20250424162633.390748816@goodmis.org>
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In-Reply-To: <20250424162633.390748816@goodmis.org>

On Thu, Apr 24, 2025 at 12:25:42PM -0400, Steven Rostedt wrote:
> +static int deferred_request(struct perf_event *event)
> +{
> +	struct callback_head *work = &event->pending_unwind_work;
> +	int pending;
> +	int ret;
> +
> +	if (!current->mm || !user_mode(task_pt_regs(current)))
> +		return -EINVAL;
> +
> +	if (in_nmi())
> +		return deferred_request_nmi(event);
> +
> +	guard(irqsave)();
> +
> +        /* callback already pending? */
> +        pending = READ_ONCE(event->pending_unwind_callback);
> +        if (pending)
> +                return 1;
> +
> +        /* Claim the work unless an NMI just now swooped in to do so. */
> +        if (!try_cmpxchg(&event->pending_unwind_callback, &pending, 1))
> +                return 1;
> +
> +        /* The work has been claimed, now schedule it. */
> +        ret = task_work_add(current, work, TWA_RESUME);
> +        if (WARN_ON_ONCE(ret)) {
> +                WRITE_ONCE(event->pending_unwind_callback, 0);
> +                return ret;
> +        }
> +	return 0;
> +}

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