Commit 8882135b authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf: Fix owner-list vs exit 



Oleg noticed that a perf-fd keeping a reference on the creating task
leads to a few funny side effects.

There's two different aspects to this:

  - kernel based perf-events, these should not take out
    a reference on the creating task and appear on the task's
    event list since they're not bound to fds nor visible
    to userspace.

  - fork() and pthread_create(), these can lead to the creating
    task dying (and thus the task's event-list becomming useless)
    but keeping the list and ref alive until the event is closed.

Combined they lead to malfunction of the ptrace hw_tracepoints.

Cure this by not considering kernel based perf_events for the
owner-list and destroying the owner-list when the owner dies.

Reported-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
LKML-Reference: <1289576883.2084.286.camel@laptop>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent fcf48a72

kernel/perf_event.c

+51 −12
Original line number Diff line number Diff line
@@ -2235,11 +2235,6 @@ int perf_event_release_kernel(struct perf_event *event) 
	raw_spin_unlock_irq(&ctx->lock);

	mutex_unlock(&ctx->mutex);



	mutex_lock(&event->owner->perf_event_mutex);

	list_del_init(&event->owner_entry);

	mutex_unlock(&event->owner->perf_event_mutex);

	put_task_struct(event->owner);



	free_event(event);



	return 0;
@@ -2252,9 +2247,43 @@ EXPORT_SYMBOL_GPL(perf_event_release_kernel); 
static int perf_release(struct inode *inode, struct file *file)

{

	struct perf_event *event = file->private_data;

	struct task_struct *owner;



	file->private_data = NULL;



	rcu_read_lock();

	owner = ACCESS_ONCE(event->owner);

	/*

	 * Matches the smp_wmb() in perf_event_exit_task(). If we observe

	 * !owner it means the list deletion is complete and we can indeed

	 * free this event, otherwise we need to serialize on

	 * owner->perf_event_mutex.

	 */

	smp_read_barrier_depends();

	if (owner) {

		/*

		 * Since delayed_put_task_struct() also drops the last

		 * task reference we can safely take a new reference

		 * while holding the rcu_read_lock().

		 */

		get_task_struct(owner);

	}

	rcu_read_unlock();



	if (owner) {

		mutex_lock(&owner->perf_event_mutex);

		/*

		 * We have to re-check the event->owner field, if it is cleared

		 * we raced with perf_event_exit_task(), acquiring the mutex

		 * ensured they're done, and we can proceed with freeing the

		 * event.

		 */

		if (event->owner)

			list_del_init(&event->owner_entry);

		mutex_unlock(&owner->perf_event_mutex);

		put_task_struct(owner);

	}



	return perf_event_release_kernel(event);

}
@@ -5678,7 +5707,7 @@ SYSCALL_DEFINE5(perf_event_open, 
	mutex_unlock(&ctx->mutex);



	event->owner = current;

	get_task_struct(current);



	mutex_lock(&current->perf_event_mutex);

	list_add_tail(&event->owner_entry, &current->perf_event_list);

	mutex_unlock(&current->perf_event_mutex);
@@ -5746,12 +5775,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu, 
	++ctx->generation;

	mutex_unlock(&ctx->mutex);



	event->owner = current;

	get_task_struct(current);

	mutex_lock(&current->perf_event_mutex);

	list_add_tail(&event->owner_entry, &current->perf_event_list);

	mutex_unlock(&current->perf_event_mutex);



	return event;



err_free:
@@ -5902,8 +5925,24 @@ again: 
 */

void perf_event_exit_task(struct task_struct *child)

{

	struct perf_event *event, *tmp;

	int ctxn;



	mutex_lock(&child->perf_event_mutex);

	list_for_each_entry_safe(event, tmp, &child->perf_event_list,

				 owner_entry) {

		list_del_init(&event->owner_entry);



		/*

		 * Ensure the list deletion is visible before we clear

		 * the owner, closes a race against perf_release() where

		 * we need to serialize on the owner->perf_event_mutex.

		 */

		smp_wmb();

		event->owner = NULL;

	}

	mutex_unlock(&child->perf_event_mutex);



	for_each_task_context_nr(ctxn)

		perf_event_exit_task_context(child, ctxn);

}

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