[PATCH v5 6/7] perf: Don't disable preemption in perf_pending_task().

From: Sebastian Andrzej Siewior
Date: Thu Jul 04 2024 - 13:05:31 EST


perf_pending_task() is invoked in task context and disables preemption
because perf_swevent_get_recursion_context() used to access per-CPU
variables. The other reason is to create a RCU read section while
accessing the perf_event.

The recursion counter is no longer a per-CPU accounter so disabling
preemption is no longer required. The RCU section is needed and must be
created explicit.

Replace the preemption-disable section with a explicit RCU-read section.

Tested-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
kernel/events/core.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1a26a9c33306a..67f5aab933c81 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5208,10 +5208,9 @@ static void perf_pending_task_sync(struct perf_event *event)
}

/*
- * All accesses related to the event are within the same
- * non-preemptible section in perf_pending_task(). The RCU
- * grace period before the event is freed will make sure all
- * those accesses are complete by then.
+ * All accesses related to the event are within the same RCU section in
+ * perf_pending_task(). The RCU grace period before the event is freed
+ * will make sure all those accesses are complete by then.
*/
rcuwait_wait_event(&event->pending_work_wait, !event->pending_work, TASK_UNINTERRUPTIBLE);
}
@@ -6842,7 +6841,7 @@ static void perf_pending_task(struct callback_head *head)
* critical section as the ->pending_work reset. See comment in
* perf_pending_task_sync().
*/
- preempt_disable_notrace();
+ rcu_read_lock();
/*
* If we 'fail' here, that's OK, it means recursion is already disabled
* and we won't recurse 'further'.
@@ -6855,10 +6854,10 @@ static void perf_pending_task(struct callback_head *head)
local_dec(&event->ctx->nr_pending);
rcuwait_wake_up(&event->pending_work_wait);
}
+ rcu_read_unlock();

if (rctx >= 0)
perf_swevent_put_recursion_context(rctx);
- preempt_enable_notrace();
}

#ifdef CONFIG_GUEST_PERF_EVENTS
--
2.45.2