Re: [PATCH V3 03/12] perf mmap: discard 'prev' in perf_mmap__read

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Dec 21, 2017 at 10:08:45AM -0800, kan.liang@intel.com wrote:
> From: Kan Liang <kan.liang@intel.com>
> 
> 'start' and 'prev' are duplicate in perf_mmap__read()
> 
> Use 'map->prev' to replace 'start' in perf_mmap__read_*().
> 
> Suggested-by: Wang Nan <wangnan0@huawei.com>
> Signed-off-by: Kan Liang <kan.liang@intel.com>
> ---
>  tools/perf/util/mmap.c | 28 ++++++++++------------------
>  1 file changed, 10 insertions(+), 18 deletions(-)
> 
> diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
> index 3fd4f3c..a844a2f 100644
> --- a/tools/perf/util/mmap.c
> +++ b/tools/perf/util/mmap.c
> @@ -22,29 +22,27 @@ size_t perf_mmap__mmap_len(struct perf_mmap *map)
>  
>  /* When check_messup is true, 'end' must points to a good entry */
>  static union perf_event *perf_mmap__read(struct perf_mmap *map,
> -					 u64 start, u64 end, u64 *prev)
> +					 u64 *start, u64 end)
>  {
>  	unsigned char *data = map->base + page_size;
>  	union perf_event *event = NULL;
> -	int diff = end - start;
> +	int diff = end - *start;

I'd like more if the arg was pointer, like 'u64 *startp' and you
kept using the 'u64 start' value in the body.. but can't see any
technical benefit behind it to force it ;-)

jirka

>  
>  	if (diff >= (int)sizeof(event->header)) {
>  		size_t size;
>  
> -		event = (union perf_event *)&data[start & map->mask];
> +		event = (union perf_event *)&data[*start & map->mask];
>  		size = event->header.size;
>  
> -		if (size < sizeof(event->header) || diff < (int)size) {
> -			event = NULL;
> -			goto broken_event;
> -		}
> +		if (size < sizeof(event->header) || diff < (int)size)
> +			return NULL;
>  
>  		/*
>  		 * Event straddles the mmap boundary -- header should always
>  		 * be inside due to u64 alignment of output.
>  		 */
> -		if ((start & map->mask) + size != ((start + size) & map->mask)) {
> -			unsigned int offset = start;
> +		if ((*start & map->mask) + size != ((*start + size) & map->mask)) {
> +			unsigned int offset = *start;
>  			unsigned int len = min(sizeof(*event), size), cpy;
>  			void *dst = map->event_copy;
>  
> @@ -59,20 +57,15 @@ static union perf_event *perf_mmap__read(struct perf_mmap *map,
>  			event = (union perf_event *)map->event_copy;
>  		}
>  
> -		start += size;
> +		*start += size;
>  	}
>  
> -broken_event:
> -	if (prev)
> -		*prev = start;
> -
>  	return event;
>  }

SNIP