perf: Fix race in swevent hash

There's a race on CPU unplug where we free the swevent hash array
while it can still have events on. This will result in a
use-after-free which is BAD.

Simply do not free the hash array on unplug. This leaves the thing
around and no use-after-free takes place.

When the last swevent dies, we do a for_each_possible_cpu() iteration
anyway to clean these up, at which time we'll free it, so no leakage
will occur.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
static void perf_event_task(struct task_struct *task,
			      struct perf_event_context *task_ctx,
			      int new)
{
	struct perf_task_event task_event;

	if (!atomic_read(&nr_comm_events) &&
	    !atomic_read(&nr_mmap_events) &&
	    !atomic_read(&nr_task_events))
		return;

	task_event = (struct perf_task_event){
		.task	  = task,
		.task_ctx = task_ctx,
		.event_id    = {
			.header = {
				.type = new ? PERF_RECORD_FORK : PERF_RECORD_EXIT,
				.misc = 0,
				.size = sizeof(task_event.event_id),
			},
			/* .pid  */
			/* .ppid */
			/* .tid  */
			/* .ptid */
			/* .time */
		},
	};

	perf_event_aux(perf_event_task_output,
		       &task_event,
		       task_ctx);
}

static void perf_event_task(struct task_struct *task,
			      struct perf_event_context *task_ctx,
			      int new)
{
	struct perf_task_event task_event;

	if (!atomic_read(&nr_comm_events) &&
	    !atomic_read(&nr_mmap_events) &&
	    !atomic_read(&nr_task_events))
		return;

	task_event = (struct perf_task_event){
		.task	  = task,
		.task_ctx = task_ctx,
		.event_id    = {
			.header = {
				.type = new ? PERF_RECORD_FORK : PERF_RECORD_EXIT,
				.misc = 0,
				.size = sizeof(task_event.event_id),
			},
			/* .pid  */
			/* .ppid */
			/* .tid  */
			/* .ptid */
			/* .time */
		},
	};

	perf_event_aux(perf_event_task_output,
		       &task_event,
		       task_ctx);
}
