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

["Wangnan (F)" <wangnan0@huawei.com>]

On 2016/3/29 8:27, Alexei Starovoitov wrote:
> 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?

Good catch. Thank you!