Subject: [PATCH 3/4] perf: Remove perf_swevent_get_recursion_context() from perf_pending_task().
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Tue, 12 Mar 2024 19:01:51 +0100

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 <[email-masked]>
Tested-by: Arnaldo Carvalho de Melo <[email-masked]>
Reported-by: Arnaldo Carvalho de Melo <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/events/core.c |   12 ------------
 1 file changed, 12 deletions(-)

Index: linux-6.8.2-rt10/kernel/events/core.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/events/core.c
+++ linux-6.8.2-rt10/kernel/events/core.c
@@ -6785,14 +6785,6 @@ static void perf_pending_irq(struct irq_
 static void perf_pending_task(struct callback_head *head)
 {
 	struct perf_event *event = container_of(head, struct perf_event, pending_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 = perf_swevent_get_recursion_context();
 
 	if (event->pending_work) {
 		event->pending_work = 0;
@@ -6800,10 +6792,6 @@ static void perf_pending_task(struct cal
 		local_dec(&event->ctx->nr_pending);
 	}
 
-	if (rctx >= 0)
-		perf_swevent_put_recursion_context(rctx);
-	preempt_enable_notrace();
-
 	put_event(event);
 }