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: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Oleg Nesterov <oleg@redhat.com>
LKML-Reference: <1289576883.2084.286.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 51 insertions, 12 deletions

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index f818d9d2dc93..671f6c8c8a32 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -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);
 }