[PATCH] perf/core: Allow list_del during perf_event_overflow()

[Thomas Richter]

A PMU might use perf_sched_cb_inc() and perf_sched_cb_dec()
interface to get the PMU call back function pmu::sched_task
invoked at schedule in and schedule out. This is achieved
by walking along the list anchored by sched_cb_list.

The following scenario might lead to a list corruption.

   perf_pmu_sched_task()
      for_each_list_entry(..., &sched_cb_list)
      +--> __perf_pmu_sched_task()
           +--> event->pmu->sched_task())
                +--> PMU_push_sample()
                     +--> perf_event_overflow()
                          +--> __perf_event_overflow()
                               +--> pmu->stop()
                                    +--> perf_sched_cb_dec()
                                         remove entry from sched_cb_list
                                         while list node in use.

This happens when ioctl(fd, PERF_EVENT_IOC_REFRESH, xxx) has been
invoked and perf_event::event_limit hits zero.

Prevent the list corruption and convert for_each_list_entry()
to for_each_list_entry_safe().

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
---
 kernel/events/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a6c8e38a3110..cdd9518ac046 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3925,13 +3925,13 @@ static void perf_pmu_sched_task(struct task_struct *prev,
 				bool sched_in)
 {
 	struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
-	struct perf_cpu_pmu_context *cpc;
+	struct perf_cpu_pmu_context *cpc, *cpc2;
 
 	/* cpuctx->task_ctx will be handled in perf_event_context_sched_in/out */
 	if (prev == next || cpuctx->task_ctx)
 		return;
 
-	list_for_each_entry(cpc, this_cpu_ptr(&sched_cb_list), sched_cb_entry)
+	list_for_each_entry_safe(cpc, cpc2, this_cpu_ptr(&sched_cb_list), sched_cb_entry)
 		__perf_pmu_sched_task(cpc, sched_in ? next : prev, sched_in);
 }
 
-- 
2.55.0