On Fri, Mar 14, 2014 at 11:43:17PM +0100, Peter Zijlstra wrote:
> 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 :-)
Well, given that I sometimes miss things when looking through RCU code
carefuly, I guess I cannot give you too much trouble about it.
> I knew there'd be some subtlety between completed and gpnum and such :-)
Some of which I have learned about one RCU bug at a time. ;-)
> > 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
> :-)
Fair enough, let me see what I can come up with.
Thanx, Paul
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