perf/core: Order the PMU list to fix warning about unordered pmu_ctx_list

In the Linux kernel, the following vulnerability has been resolved:

perf/core: Order the PMU list to fix warning about unordered pmu_ctx_list

Syskaller triggers a warning due to prev_epc->pmu != next_epc->pmu in

perf_event_swap_task_ctx_data(). vmcore shows that two lists have the same

perf_event_pmu_context, but not in the same order.

The problem is that the order of pmu_ctx_list for the parent is impacted by

the time when an event/PMU is added. While the order for a child is

impacted by the event order in the pinned_groups and flexible_groups. So

the order of pmu_ctx_list in the parent and child may be different.

To fix this problem, insert the perf_event_pmu_context to its proper place

after iteration of the pmu_ctx_list.

The follow testcase can trigger above warning:

# perf record -e cycles --call-graph lbr -- taskset -c 3 ./a.out &

# perf stat -e cpu-clock,cs -p xxx // xxx is the pid of a.out

test.c

void main() {

int count = 0;

pid_t pid;

printf("%d running\n", getpid());

sleep(30);

printf("running\n");

pid = fork();

if (pid == -1) {

printf("fork error\n");

return;

}

if (pid == 0) {

while (1) {

count++;

}

} else {

while (1) {

count++;

}

}

}

The testcase first opens an LBR event, so it will allocate task_ctx_data,

and then open tracepoint and software events, so the parent context will

have 3 different perf_event_pmu_contexts. On inheritance, child ctx will

insert the perf_event_pmu_context in another order and the warning will

trigger.

[ mingo: Tidied up the changelog. ]