Commit 2ab9d830 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf/aux: Fix AUX buffer serialization 



Ole reported that event->mmap_mutex is strictly insufficient to
serialize the AUX buffer, add a per RB mutex to fully serialize it.

Note that in the lock order comment the perf_event::mmap_mutex order
was already wrong, that is, it nesting under mmap_lock is not new with
this patch.

Fixes: 45bfb2e5 ("perf: Add AUX area to ring buffer for raw data streams")
Reported-by: default avatarOle <ole@binarygecko.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent e240b0fd

kernel/events/core.c

+12 −6
Original line number Diff line number Diff line
@@ -1255,8 +1255,9 @@ 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
@@ -6373,12 +6374,11 @@ static void perf_mmap_close(struct vm_area_struct *vma) 
		event->pmu->event_unmapped(event, vma->vm_mm);



	/*

	 * 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.

	 * The AUX buffer is strictly a sub-buffer, serialize using aux_mutex

	 * to avoid complications.

	 */

	if (rb_has_aux(rb) && vma->vm_pgoff == rb->aux_pgoff &&

	    atomic_dec_and_mutex_lock(&rb->aux_mmap_count, &event->mmap_mutex)) {

	    atomic_dec_and_mutex_lock(&rb->aux_mmap_count, &rb->aux_mutex)) {

		/*

		 * Stop all AUX events that are writing to this buffer,

		 * so that we can free its AUX pages and corresponding PMU
@@ -6395,7 +6395,7 @@ static void perf_mmap_close(struct vm_area_struct *vma) 
		rb_free_aux(rb);

		WARN_ON_ONCE(refcount_read(&rb->aux_refcount));



		mutex_unlock(&event->mmap_mutex);

		mutex_unlock(&rb->aux_mutex);

	}



	if (atomic_dec_and_test(&rb->mmap_count))
@@ -6483,6 +6483,7 @@ 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;
@@ -6531,6 +6532,9 @@ 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);
@@ -6681,6 +6685,8 @@ 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

+1 −0
Original line number Diff line number Diff line
@@ -40,6 +40,7 @@ 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

+2 −0
Original line number Diff line number Diff line
@@ -337,6 +337,8 @@ 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)