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Date: Mon, 28 Mar 2016 17:27:26 -0700
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        He Kuang <hekuang@huawei.com>, Jiri Olsa <jolsa@kernel.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Namhyung Kim <namhyung@kernel.org>, pi3orama@163.com,
        Zefan Li <lizefan@huawei.com>
Subject: Re: [PATCH 1/4] perf core: Introduce new ioctl options to pause and
 resume ring buffer
Message-ID: <20160329002724.GB31198@ast-mbp.thefacebook.com>
References: <1459147292-239310-1-git-send-email-wangnan0@huawei.com>
 <1459147292-239310-2-git-send-email-wangnan0@huawei.com>
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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@huawei.com>
> Cc: He Kuang <hekuang@huawei.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Zefan Li <lizefan@huawei.com>
> Cc: pi3orama@163.com
> ---
>  include/uapi/linux/perf_event.h |  1 +
>  kernel/events/core.c            | 13 +++++++++++++
>  kernel/events/internal.h        | 11 +++++++++++
>  kernel/events/ring_buffer.c     |  7 ++++++-
>  4 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 1afe962..a3c1903 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -401,6 +401,7 @@ struct perf_event_attr {
>  #define PERF_EVENT_IOC_SET_FILTER	_IOW('$', 6, char *)
>  #define PERF_EVENT_IOC_ID		_IOR('$', 7, __u64 *)
>  #define PERF_EVENT_IOC_SET_BPF		_IOW('$', 8, __u32)
> +#define PERF_EVENT_IOC_PAUSE_OUTPUT	_IOW('$', 9, __u32)
>  
>  enum perf_event_ioc_flags {
>  	PERF_IOC_FLAG_GROUP		= 1U << 0,
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index de24fbc..cb47da3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4341,6 +4341,19 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
>  	case PERF_EVENT_IOC_SET_BPF:
>  		return perf_event_set_bpf_prog(event, arg);
>  
> +	case PERF_EVENT_IOC_PAUSE_OUTPUT: {
> +		struct ring_buffer *rb;
> +
> +		rcu_read_lock();
> +		rb = rcu_dereference(event->rb);
> +		if (!event->rb) {

should have been 'if (!rb)', right?