perf_mmap_close和ioctl存在条件竞争，导致在perf_mmap时进入死循环

CPU1 CPU2

perf_mmap_close(e2)
              if (atomic_dec_and_test(&e2->rb->mmap_count)) // 1 - > 0
                detach_rest = true

ioctl(e1, IOC_SET_OUTPUT, e2)
                                                      perf_event_set_output(e1, e2)

...
              list_for_each_entry_rcu(e, &e2->rb->event_list, rb_entry)
                ring_buffer_attach(e, NULL);
                // e1 isn't yet added and
                // therefore not detached

ring_buffer_attach(e1, e2->rb)
                                                          list_add_rcu(&e1->rb_entry,
                                                                       &e2->rb->event_list)

After this; e1 is attached to an unmapped rb and a subsequent
    perf_mmap() will loop forever more:

again:
                    mutex_lock(&e->mmap_mutex);
                    if (event->rb) {
                            ...
                            if (!atomic_inc_not_zero(&e->rb->mmap_count)) {
                                    ...
                                    mutex_unlock(&e->mmap_mutex);
                                    goto again;
                            }
                    }

回合主线修复补丁：68e3c69803dada336893640110cb87221bb01dcf

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 80782cddb1da..d2b354991bf5 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6253,10 +6253,10 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)

                if (!atomic_inc_not_zero(&event->rb->mmap_count)) {
                        /*
-                        * Raced against perf_mmap_close() through
-                        * perf_event_set_output(). Try again, hope for better
-                        * luck.
+                        * Raced against perf_mmap_close(); remove the
+                        * event and try again.
                         */
+                       ring_buffer_attach(event, NULL);
                        mutex_unlock(&event->mmap_mutex);
                        goto again;
                }
@@ -11825,14 +11825,25 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
        goto out;
 }

+static void mutex_lock_double(struct mutex *a, struct mutex *b)
+{
+       if (b < a)
+               swap(a, b);
+
+       mutex_lock(a);
+       mutex_lock_nested(b, SINGLE_DEPTH_NESTING);
+}
+
 static int
 perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
 {
        struct perf_buffer *rb = NULL;
        int ret = -EINVAL;

-       if (!output_event)
+       if (!output_event) {
+               mutex_lock(&event->mmap_mutex);
                goto set;
+       }

在perf_mmap时进入循环时，将ringbuffer attch成NULL，以跳出循环

合入修复补丁后，问题不再复现：
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