Messages in this thread | | | Date | Fri, 14 Mar 2014 16:02:31 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] [RFC] perf: Fix a race between ring_buffer_detach() and ring_buffer_wakeup() |
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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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