On Fri, Jun 25, 2010 at 02:05:24AM +1000, Nick Piggin wrote: > On Thu, Jun 24, 2010 at 08:32:18AM -0700, Paul E. McKenney wrote: > > On Fri, Jun 25, 2010 at 01:07:06AM +1000, Nick Piggin wrote: > > > On Thu, Jun 24, 2010 at 10:44:22AM +0200, Peter Zijlstra wrote: > > > > On Thu, 2010-06-24 at 13:02 +1000, npiggin@xxxxxxx wrote: > > > > > Use RCU property of dcache to simplify locking in some places where we > > > > > take d_parent and d_lock. > > > > > > > > > > Comment: don't need rcu_deref because we take the spinlock and recheck it. > > > > > > > > But does the LOCK barrier imply a DATA DEPENDENCY barrier? (It does on > > > > x86, and the compiler barrier implied by spin_lock() suffices to replace > > > > ACCESS_ONCE()). > > > > > > Well the dependency we care about is from loading the parent pointer > > > to acquiring its spinlock. But we can't possibly have stale data given > > > to the spin lock operation itself because it is a RMW. > > > > As long as you check for the structure being valid after acquiring the > > lock, I agree. Otherwise, I would be concerned about the following > > sequence of events: > > > > 1. CPU 0 picks up a pointer to a given data element. > > > > 2. CPU 1 removes this element from the list, drops any locks that > > it might have, and starts waiting for a grace period to > > elapse. > > > > 3. CPU 0 acquires the lock, does some operation that would > > be appropriate had the element not been removed, then > > releases the lock. > > > > 4. After the grace period, CPU 1 frees the element, negating > > CPU 0's hard work. > > > > The usual approach is to have a "deleted" flag or some such in the > > element that CPU 0 would set when removing the element and that CPU 1 > > would check after acquiring the lock. Which you might well already > > be doing! ;-) > > Thanks, yep it's done under RCU, and after taking the lock it rechecks > to see that it is still reachable by the same pointer (and if not, > unlocks and retries) so it should be fine. Very good!!! ;-) Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
