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    SubjectRe: [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@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?

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