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 :-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/