Re: [PATCH 2/9] perf_counter: fix update_userpage()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2009-04-02 at 10:50 +0200, Peter Zijlstra wrote:
> On Sun, 2009-03-29 at 11:24 +1100, Paul Mackerras wrote:
> 
> > > --- linux-2.6.orig/include/linux/perf_counter.h
> > > +++ linux-2.6/include/linux/perf_counter.h
> > > @@ -160,10 +160,45 @@ struct perf_counter_hw_event {
> > >  struct perf_counter_mmap_page {
> > >  	__u32	version;		/* version number of this structure */
> > >  	__u32	compat_version;		/* lowest version this is compat with */
> > > +
> > > +	/*
> > > +	 * Bits needed to read the hw counters in user-space.
> > > +	 *
> > > +	 * The index and offset should be read atomically using the seqlock:
> > > +	 *
> > > +	 *   __u32 seq, index;
> > > +	 *   __s64 offset;
> > > +	 *
> > > +	 * again:
> > > +	 *   rmb();
> > > +	 *   seq = pc->lock;
> > > +	 *
> > > +	 *   if (unlikely(seq & 1)) {
> > > +	 *     cpu_relax();
> > > +	 *     goto again;
> > > +	 *   }
> > > +	 *
> > > +	 *   index = pc->index;
> > > +	 *   offset = pc->offset;
> > > +	 *
> > > +	 *   rmb();
> > > +	 *   if (pc->lock != seq)
> > > +	 *     goto again;
> > > +	 *
> > > +	 * After this, index contains architecture specific counter index + 1,
> > > +	 * so that 0 means unavailable, offset contains the value to be added
> > > +	 * to the result of the raw timer read to obtain this counter's value.
> > > +	 */
> > >  	__u32	lock;			/* seqlock for synchronization */
> > >  	__u32	index;			/* hardware counter identifier */
> > >  	__s64	offset;			/* add to hardware counter value */
> > 
> > I think we can simplify this (in a follow-on patch).
> > 
> > It has occurred to me that we don't need to do all this on the
> > userspace side, because we are necessarily reading and writing these
> > fields on the same CPU.  If the reader and writer were on different
> > CPUs, that would make no sense since they would be accessing different
> > hardware counter registers.
> > 
> > That means that we don't need any CPU memory barriers on either side.
> > All the kernel needs to do is to increment `lock' when it updates
> > things, and the user side can be:
> > 
> > 	do {
> > 		seq = pc->lock;
> > 		index = pc->index;
> > 		offset = pc->offset;
> > 		barrier();
> > 	} while (pc->lock != seq);
> > 
> > and all that's needed is a compiler barrier to stop the compiler from
> > optimizing too much.
> 
> Can this work at all?
> 
> I mean, user-space could get preempted/rescheduled after we read the
> mmap() data using that seqlock and before we actually did the read-pmc
> bit.
> 
> In that case, the counter can have changed underneath us and we're
> reading rubbish.

The below might work:

  u32 seq;
  s64 count;

again:
  seq = pc->lock;

  if (unlikely(seq & 1)) {
    cpu_relax();
    goto again;
  }

  count = pmc_read(pc->index);
  counter += pc->offset;

  barrier();
  if (pc->lock != seq)
    goto again;