RE: [PATCH V3 02/12] perf mmap: factor out function to find ringbuffer position

[Liang, Kan]

> On Thu, Dec 21, 2017 at 10:08:44AM -0800, kan.liang@xxxxxxxxx wrote:
> 
> SNIP
> 
> > +/*
> > + * Report the start and end of the available data in ringbuffer
> > + */
> > +int perf_mmap__read_init(struct perf_mmap *map, bool overwrite,
> > +			 u64 *start, u64 *end)
> >  {
> > -	u64 head = perf_mmap__read_head(md);
> > -	u64 old = md->prev;
> > -	u64 end = head, start = old;
> > -	unsigned char *data = md->base + page_size;
> > +	u64 head = perf_mmap__read_head(map);
> > +	u64 old = map->prev;
> > +	unsigned char *data = map->base + page_size;
> >  	unsigned long size;
> > -	void *buf;
> > -	int rc = 0;
> >
> > -	start = overwrite ? head : old;
> > -	end = overwrite ? old : head;
> > +	/*
> > +	 * Check if event was unmapped due to a POLLHUP/POLLERR.
> > +	 */
> > +	if (!refcount_read(&map->refcnt))
> > +		return -EINVAL;
> >
> > -	if (start == end)
> > -		return 0;
> > +	*start = overwrite ? head : old;
> > +	*end = overwrite ? old : head;
> >
> > -	size = end - start;
> > -	if (size > (unsigned long)(md->mask) + 1) {
> > +	if (*start == *end)
> > +		return -EAGAIN;
> > +
> > +	size = *end - *start;
> > +	if (size > (unsigned long)(map->mask) + 1) {
> >  		if (!overwrite) {
> >  			WARN_ONCE(1, "failed to keep up with mmap data.
> (warn only once)\n");
> >
> 
> I know you did not change this, but is this leg even possible
> in !overwrite mode? I think kernel will throw away the data,
> keep the head and wait for tail to be read by user..

Right, it should not happen in !overwrite mode. I guess it's just
sanity check.
It should not bring any problems.
I think I will still keep it for V4 if no objection? 

Thanks,
Kan

> 
> jirka
> 
> > -			md->prev = head;
> > -			perf_mmap__consume(md, overwrite);
> > -			return 0;
> > +			map->prev = head;
> > +			perf_mmap__consume(map, overwrite);
> > +			return -EAGAIN;
> 
> SNIP