linux-kernel - Re: [PATCH V3 02/12] perf mmap: factor out function to find ringbuffer position

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Date:   Fri, 12 Jan 2018 11:52:00 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     "acme@...nel.org" <acme@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "wangnan0@...wei.com" <wangnan0@...wei.com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "yao.jin@...ux.intel.com" <yao.jin@...ux.intel.com>
Subject: Re: [PATCH V3 02/12] perf mmap: factor out function to find
 ringbuffer position

On Thu, Jan 11, 2018 at 09:26:28PM +0000, Liang, Kan wrote:
>  
> > On Thu, Dec 21, 2017 at 10:08:44AM -0800, kan.liang@...el.com 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? 

ok

jirka