linux-kernel - [tip: perf/urgent] perf/core: Fix system hang caused by cpu-clock usage

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Message-ID: <176216388242.2601451.15376301096762514312.tip-bot2@tip-bot2>
Date: Mon, 03 Nov 2025 09:58:02 -0000
From: "tip-bot2 for Dapeng Mi" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Octavia Togami <octavia.togami@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Dapeng Mi <dapeng1.mi@...ux.intel.com>,
 Ingo Molnar <mingo@...nel.org>, x86@...nel.org, linux-kernel@...r.kernel.org
Subject:
 [tip: perf/urgent] perf/core: Fix system hang caused by cpu-clock usage

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     e061eb22817cb15e65b91e46d3fa8cd5ae60f9f4
Gitweb:        https://git.kernel.org/tip/e061eb22817cb15e65b91e46d3fa8cd5ae60f9f4
Author:        Dapeng Mi <dapeng1.mi@...ux.intel.com>
AuthorDate:    Wed, 15 Oct 2025 13:18:28 +08:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 03 Nov 2025 10:52:10 +01:00

perf/core: Fix system hang caused by cpu-clock usage

cpu-clock usage by the async-profiler tool can trigger a system hang,
which got bisected back to the following commit by Octavia Togami:

  18dbcbfabfff ("perf: Fix the POLL_HUP delivery breakage") causes this issue

The root cause of the hang is that cpu-clock is a special type of SW
event which relies on hrtimers. The __perf_event_overflow() callback
is invoked from the hrtimer handler for cpu-clock events, and
__perf_event_overflow() tries to call cpu_clock_event_stop()
to stop the event, which calls htimer_cancel() to cancel the hrtimer.

But that's a recursion into the hrtimer code from a hrtimer handler,
which (unsurprisingly) deadlocks.

To fix this bug, use hrtimer_try_to_cancel() instead, and set
the PERF_HES_STOPPED flag, which causes perf_swevent_hrtimer()
to stop the event once it sees the PERF_HES_STOPPED flag.

[ mingo: Fixed the comments and improved the changelog. ]

Closes: https://lore.kernel.org/all/CAHPNGSQpXEopYreir+uDDEbtXTBvBvi8c6fYXJvceqtgTPao3Q@mail.gmail.com/
Fixes: 18dbcbfabfff ("perf: Fix the POLL_HUP delivery breakage")
Reported-by: Octavia Togami <octavia.togami@...il.com>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Tested-by: Octavia Togami <octavia.togami@...il.com>
Link: https://github.com/lucko/spark/issues/530
Link: https://patch.msgid.link/20251015051828.12809-1-dapeng1.mi@linux.intel.com
---
 kernel/events/core.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 177e57c..1fd347d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11773,7 +11773,8 @@ static enum hrtimer_restart perf_swevent_hrtimer(struct hrtimer *hrtimer)
 
 	event = container_of(hrtimer, struct perf_event, hw.hrtimer);
 
-	if (event->state != PERF_EVENT_STATE_ACTIVE)
+	if (event->state != PERF_EVENT_STATE_ACTIVE ||
+	    event->hw.state & PERF_HES_STOPPED)
 		return HRTIMER_NORESTART;
 
 	event->pmu->read(event);
@@ -11819,15 +11820,20 @@ static void perf_swevent_cancel_hrtimer(struct perf_event *event)
 	struct hw_perf_event *hwc = &event->hw;
 
 	/*
-	 * The throttle can be triggered in the hrtimer handler.
-	 * The HRTIMER_NORESTART should be used to stop the timer,
-	 * rather than hrtimer_cancel(). See perf_swevent_hrtimer()
+	 * Careful: this function can be triggered in the hrtimer handler,
+	 * for cpu-clock events, so hrtimer_cancel() would cause a
+	 * deadlock.
+	 *
+	 * So use hrtimer_try_to_cancel() to try to stop the hrtimer,
+	 * and the cpu-clock handler also sets the PERF_HES_STOPPED flag,
+	 * which guarantees that perf_swevent_hrtimer() will stop the
+	 * hrtimer once it sees the PERF_HES_STOPPED flag.
 	 */
 	if (is_sampling_event(event) && (hwc->interrupts != MAX_INTERRUPTS)) {
 		ktime_t remaining = hrtimer_get_remaining(&hwc->hrtimer);
 		local64_set(&hwc->period_left, ktime_to_ns(remaining));
 
-		hrtimer_cancel(&hwc->hrtimer);
+		hrtimer_try_to_cancel(&hwc->hrtimer);
 	}
 }
 
@@ -11871,12 +11877,14 @@ static void cpu_clock_event_update(struct perf_event *event)
 
 static void cpu_clock_event_start(struct perf_event *event, int flags)
 {
+	event->hw.state = 0;
 	local64_set(&event->hw.prev_count, local_clock());
 	perf_swevent_start_hrtimer(event);
 }
 
 static void cpu_clock_event_stop(struct perf_event *event, int flags)
 {
+	event->hw.state = PERF_HES_STOPPED;
 	perf_swevent_cancel_hrtimer(event);
 	if (flags & PERF_EF_UPDATE)
 		cpu_clock_event_update(event);
@@ -11950,12 +11958,14 @@ static void task_clock_event_update(struct perf_event *event, u64 now)
 
 static void task_clock_event_start(struct perf_event *event, int flags)
 {
+	event->hw.state = 0;
 	local64_set(&event->hw.prev_count, event->ctx->time);
 	perf_swevent_start_hrtimer(event);
 }
 
 static void task_clock_event_stop(struct perf_event *event, int flags)
 {
+	event->hw.state = PERF_HES_STOPPED;
 	perf_swevent_cancel_hrtimer(event);
 	if (flags & PERF_EF_UPDATE)
 		task_clock_event_update(event, event->ctx->time);