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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: linux-perf-users@vger.kernel.org,
	linux-kernel@vger.kernel.org
Cc: Adrian Hunter <adrian.hunter@intel.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Ian Rogers <irogers@google.com>,
	Ingo Molnar <mingo@redhat.com>,
	Jiri Olsa <jolsa@kernel.org>,
	Marco Elver <elver@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: [PATCH v3 3/4] perf: Remove perf_swevent_get_recursion_context() from perf_pending_task().
Date: Fri, 22 Mar 2024 07:48:23 +0100
Message-ID: <20240322065208.60456-4-bigeasy@linutronix.de>
In-Reply-To: <20240322065208.60456-1-bigeasy@linutronix.de>
References: <20240322065208.60456-1-bigeasy@linutronix.de>
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perf_swevent_get_recursion_context() is supposed to avoid recursion.
This requires to remain on the same CPU in order to decrement/ increment
the same counter. This is done by using preempt_disable(). Having
preemption disabled while sending a signal leads to locking problems on
PREEMPT_RT because sighand, a spinlock_t, becomes a sleeping lock.

This callback runs in task context and currently delivers only a signal
to "itself". Any kind of recusrion protection in this context is not
required.

Remove recursion protection in perf_pending_task().

Tested-by: Marco Elver <elver@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/events/core.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index e0b2da8de485f..5400f7ed2f98b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6785,14 +6785,6 @@ static void perf_pending_irq(struct irq_work *entry)
 static void perf_pending_task(struct callback_head *head)
 {
 	struct perf_event *event =3D container_of(head, struct perf_event, pendin=
g_task);
-	int rctx;
-
-	/*
-	 * If we 'fail' here, that's OK, it means recursion is already disabled
-	 * and we won't recurse 'further'.
-	 */
-	preempt_disable_notrace();
-	rctx =3D perf_swevent_get_recursion_context();
=20
 	if (event->pending_work) {
 		event->pending_work =3D 0;
@@ -6800,10 +6792,6 @@ static void perf_pending_task(struct callback_head *=
head)
 		local_dec(&event->ctx->nr_pending);
 	}
=20
-	if (rctx >=3D 0)
-		perf_swevent_put_recursion_context(rctx);
-	preempt_enable_notrace();
-
 	put_event(event);
 }
=20
--=20
2.43.0