[PATCH] perf: fix racy update of cpc->hrtimer_interval

[Benjamin Segall <bsegall@google.com>]

Updates to cpc->hrtimer_interval can race with reads either in the
hrtimer itself, or other calls to perf_mux_hrtimer_restart. This is not
exactly expected to be a common config change, but it's easy enough to
fix.

Since there's already an IPI to each cpu to restart the timer, and all
reads are with irqs disabled, just move the update onto the target cpu.

Fixes: 62b856397927 ("perf: Add sysfs entry to adjust multiplexing interval per PMU")
Signed-off-by: Ben Segall <bsegall@google.com>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/xm265wykhyrv.fsf%40google.com
---
 kernel/events/core.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 457d75ea71420..3415e4e3c26db 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1330,20 +1330,25 @@ static int perf_mux_hrtimer_restart(struct perf_cpu_pmu_context *cpc)
 	raw_spin_unlock_irqrestore(&cpc->hrtimer_lock, flags);
 
 	return 0;
 }
 
-static int perf_mux_hrtimer_restart_ipi(void *arg)
-{
-	return perf_mux_hrtimer_restart(arg);
-}
-
 static __always_inline struct perf_cpu_pmu_context *this_cpc(struct pmu *pmu)
 {
 	return *this_cpu_ptr(pmu->cpu_pmu_context);
 }
 
+static int perf_mux_hrtimer_interval_ipi(void *arg)
+{
+	struct pmu *pmu = arg;
+	struct perf_cpu_pmu_context *cpc = this_cpc(pmu);
+	int ms = pmu->hrtimer_interval_ms;
+
+	cpc->hrtimer_interval = ns_to_ktime(NSEC_PER_MSEC * ms);
+	return perf_mux_hrtimer_restart(cpc);
+}
+
 void perf_pmu_disable(struct pmu *pmu)
 {
 	int *count = &this_cpc(pmu)->pmu_disable_count;
 	if (!(*count)++)
 		pmu->pmu_disable(pmu);
@@ -12763,17 +12768,12 @@ perf_event_mux_interval_ms_store(struct device *dev,
 	mutex_lock(&mux_interval_mutex);
 	pmu->hrtimer_interval_ms = timer;
 
 	/* update all cpuctx for this PMU */
 	cpus_read_lock();
-	for_each_online_cpu(cpu) {
-		struct perf_cpu_pmu_context *cpc;
-		cpc = *per_cpu_ptr(pmu->cpu_pmu_context, cpu);
-		cpc->hrtimer_interval = ns_to_ktime(NSEC_PER_MSEC * timer);
-
-		cpu_function_call(cpu, perf_mux_hrtimer_restart_ipi, cpc);
-	}
+	for_each_online_cpu(cpu)
+		cpu_function_call(cpu, perf_mux_hrtimer_interval_ipi, pmu);
 	cpus_read_unlock();
 	mutex_unlock(&mux_interval_mutex);
 
 	return count;
 }
-- 
2.56.0.385.gac671c819a-goog