linux-kernel - Re: [PATCH 1/4] perf core: Introduce new ioctl options to pause and resume ring buffer

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Date:	Tue, 29 Mar 2016 14:54:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	Alexei Starovoitov <ast@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	He Kuang <hekuang@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>, pi3orama@....com,
	Zefan Li <lizefan@...wei.com>
Subject: Re: [PATCH 1/4] perf core: Introduce new ioctl options to pause and
 resume ring buffer

On Mon, Mar 28, 2016 at 06:41:29AM +0000, Wang Nan wrote:
> Add new ioctl() to pause/resume ring-buffer output.
> 
> In some situations we want to read from ring buffer only when we
> ensure nothing can write to the ring buffer during reading. Without
> this patch we have to turn off all events attached to this ring buffer
> to achieve this.
> 
> This patch is for supporting overwrite ring buffer. Following
> commits will introduce new methods support reading from overwrite ring
> buffer. Before reading, caller must ensure the ring buffer is frozen, or
> the reading is unreliable.
> 
> Signed-off-by: Wang Nan <wangnan0@...wei.com>

I made the below changes.

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4346,7 +4346,7 @@ static long _perf_ioctl(struct perf_even
 
 		rcu_read_lock();
 		rb = rcu_dereference(event->rb);
-		if (!event->rb) {
+		if (!event->rb || !event->nr_pages) {
 			rcu_read_unlock();
 			return -EINVAL;
 		}
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -66,9 +66,7 @@ static inline void rb_free_rcu(struct rc
 	rb_free(rb);
 }
 
-static inline void
-rb_toggle_paused(struct ring_buffer *rb,
-		 bool pause)
+static inline void rb_toggle_paused(struct ring_buffer *rb, bool pause)
 {
 	if (!pause && rb->nr_pages)
 		rb->paused = 0;
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -248,7 +248,12 @@ ring_buffer_init(struct ring_buffer *rb,
 	spin_lock_init(&rb->event_lock);
 	init_irq_work(&rb->irq_work, rb_irq_work);
 
-	rb->paused = rb->nr_pages ? 0 : 1;
+	/*
+	 * perf_output_begin() only checks rb->paused, therefore
+	 * rb->paused must be true if we have no pages for output.
+	 */
+	if (!rb->nr_pages)
+		rb->paused = 1;
 }
 
 static void ring_buffer_put_async(struct ring_buffer *rb)