Re: [PATCH] [RFC] perf: Fix a race between ring_buffer_detach() and ring_buffer_wakeup()

Peter Zijlstra
2014-03-18T08:26:28Z

On Mon, Mar 17, 2014 at 07:45:22PM -0700, Paul E. McKenney wrote:
> > > Yep! I will risk an ASCII diagram:
> > >
> > >
> > > 3: +----gpnum----+-- ...
> > > | |
> > > 2: +----gpnum----+-------+--completed--+
> > > | |
> > > 1: +----gpnum----+-------+--completed--+
> > > | |
> > > 0: +-----+--completed--+
> > >
> > >
> > > A full RCU grace period happens between a pair of "|"s on the same line.
> > > By inspection, if your snapshot of ->gpnum is greater than the current
> > > value of ->completed, a grace period has passed.
> >
> > OK, so I get the > part, but I'm not sure I get the = part of the above.
> > The way I read the diagram, when completed matches gpnum the grace
> > period is done and we don't have to wait anymore.
>
> Absolutely not! Let's try laying out the scenario:
>
> 1. Someone calls get_state_synchronize_rcu() when ->gpnum==->completed==0.
> It returns zero.
>
> 2. That someone immediately calls cond_synchronize_rcu(). Nothing
> has changed, so oldstate==newstate==0.
>
> We had better call synchronize_rcu() in this case!!!

> Make sense?

Yes, should have seen that! Thanks for bearing with me on this.

> ------------------------------------------------------------------------
>
> rcu: Provide grace-period piggybacking API
>
> The following pattern is currently not well supported by RCU:
>
> 1. Make data element inaccessible to RCU readers.
>
> 2. Do work that probably lasts for more than one grace period.
>
> 3. Do something to make sure RCU readers in flight before #1 above
> have completed.
>
> Here are some things that could currently be done:
>
> a. Do a synchronize_rcu() unconditionally at either #1 or #3 above.
> This works, but imposes needless work and latency.
>
> b. Post an RCU callback at #1 above that does a wakeup, then
> wait for the wakeup at #3. This works well, but likely results
> in an extra unneeded grace period. Open-coding this is also
> a bit more semi-tricky code than would be good.
>
> This commit therefore adds get_state_synchronize_rcu() and
> cond_synchronize_rcu() APIs. Call get_state_synchronize_rcu() at #1
> above and pass its return value to cond_synchronize_rcu() at #3 above.
> This results in a call to synchronize_rcu() if no grace period has
> elapsed between #1 and #3, but requires only a load, comparison, and
> memory barrier if a full grace period did elapse.
>
> Requested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Thanks!

Acked-by: Peter Zijlstra <peterz@infradead.org>