Revert "perf/aux: Fix AUX buffer serialization"

This reverts commit 52d13d224fdf1299c8b642807fa1ea14d693f5ff which is
commit 2ab9d830262c132ab5db2f571003d80850d56b2a upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ia2a0469ba8f50d54ced5d818f36c8d07f977f1a0
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

kernel/events/core.c

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6309226..7e427f1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1367,9 +1367,8 @@ static void put_ctx(struct perf_event_context *ctx)
  *	  perf_event_context::mutex
  *	    perf_event::child_mutex;
  *	      perf_event_context::lock
+ *	    perf_event::mmap_mutex
  *	    mmap_lock
- *	      perf_event::mmap_mutex
- *	        perf_buffer::aux_mutex
  *	      perf_addr_filters_head::lock
  *
  *    cpu_hotplug_lock
@@ -6207,11 +6206,12 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 		event->pmu->event_unmapped(event, vma->vm_mm);
 
 	/*
-	 * The AUX buffer is strictly a sub-buffer, serialize using aux_mutex
-	 * to avoid complications.
+	 * rb->aux_mmap_count will always drop before rb->mmap_count and
+	 * event->mmap_count, so it is ok to use event->mmap_mutex to
+	 * serialize with perf_mmap here.
 	 */
 	if (rb_has_aux(rb) && vma->vm_pgoff == rb->aux_pgoff &&
-	    atomic_dec_and_mutex_lock(&rb->aux_mmap_count, &rb->aux_mutex)) {
+	    atomic_dec_and_mutex_lock(&rb->aux_mmap_count, &event->mmap_mutex)) {
 		/*
 		 * Stop all AUX events that are writing to this buffer,
 		 * so that we can free its AUX pages and corresponding PMU
@@ -6228,7 +6228,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 		rb_free_aux(rb);
 		WARN_ON_ONCE(refcount_read(&rb->aux_refcount));
 
-		mutex_unlock(&rb->aux_mutex);
+		mutex_unlock(&event->mmap_mutex);
 	}
 
 	if (atomic_dec_and_test(&rb->mmap_count))
@@ -6316,7 +6316,6 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	struct perf_event *event = file->private_data;
 	unsigned long user_locked, user_lock_limit;
 	struct user_struct *user = current_user();
-	struct mutex *aux_mutex = NULL;
 	struct perf_buffer *rb = NULL;
 	unsigned long locked, lock_limit;
 	unsigned long vma_size;
@@ -6365,9 +6364,6 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 		if (!rb)
 			goto aux_unlock;
 
-		aux_mutex = &rb->aux_mutex;
-		mutex_lock(aux_mutex);
-
 		aux_offset = READ_ONCE(rb->user_page->aux_offset);
 		aux_size = READ_ONCE(rb->user_page->aux_size);
 
@@ -6518,8 +6514,6 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 		atomic_dec(&rb->mmap_count);
 	}
 aux_unlock:
-	if (aux_mutex)
-		mutex_unlock(aux_mutex);
 	mutex_unlock(&event->mmap_mutex);
 
 	/*

kernel/events/internal.h

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index f376b057..386d21c 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -40,7 +40,6 @@ struct perf_buffer {
 	struct user_struct		*mmap_user;
 
 	/* AUX area */
-	struct mutex			aux_mutex;
 	long				aux_head;
 	unsigned int			aux_nest;
 	long				aux_wakeup;	/* last aux_watermark boundary crossed by aux_head */

kernel/events/ring_buffer.c

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 35b60d9..b5d5aa5 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -329,8 +329,6 @@ ring_buffer_init(struct perf_buffer *rb, long watermark, int flags)
 	 */
 	if (!rb->nr_pages)
 		rb->paused = 1;
-
-	mutex_init(&rb->aux_mutex);
 }
 
 void perf_aux_output_flag(struct perf_output_handle *handle, u64 flags)