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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Mar 14, 2014 at 01:47:37PM -0700, Paul E. McKenney wrote:
> This general idea can be made to work, but it will need some
> internal-to-RCU help.  One vulnerability of the patch below is the
> following sequence of steps:
> 
> 1.	RCU has just finished a grace period, and is doing the
> 	end-of-grace-period accounting.
> 
> 2.	The code below invokes rcu_batches_completed().  Let's assume
> 	the result returned is 42.
> 
> 3.	RCU completes the end-of-grace-period accounting, and increments
> 	rcu_sched_state.completed.
> 
> 4.	The code below checks ->rcu_batches against the result from
> 	another invocation of rcu_batches_completed() and sees that
> 	the 43 is not equal to 42, so skips the synchronize_rcu().
> 
> Except that a grace period has not actually completed.  Boom!!!
> 
> The problem is that rcu_batches_completed() is only intended to give
> progress information on RCU.

Ah, I thought I was missing something when I was looking through the rcu
code in a hurry :-)

I knew there'd be some subtlety between completed and gpnum and such :-)

> What I can do is give you a pair of functions, one to take a snapshot of
> the current grace-period state (returning an unsigned long) and another
> to evaluate a previous snapshot, invoking synchronize_rcu() if there has
> not been a full grace period in the meantime.
> 
> The most straightforward approach would invoke acquiring the global
> rcu_state ->lock on each call, which I am guessing just might be
> considered to be excessive overhead.  ;-)  I should be able to decrease
> the overhead to a memory barrier on each call, and perhaps even down
> to an smp_load_acquire().  Accessing the RCU state probably costs you
> a cache miss both times.
> 
> Thoughts?

Sounds fine, the attach isn't a hotpath, so even the locked version
should be fine, but I won't keep you from making it all fancy and such
:-)