On Thu, Oct 03, 2013 at 11:03:05PM -0700, Josh Triplett wrote: > On Thu, Oct 03, 2013 at 10:29:59PM -0700, Paul E. McKenney wrote: > > On Thu, Oct 03, 2013 at 04:28:27PM -0700, Josh Triplett wrote: > > > On Thu, Oct 03, 2013 at 01:52:45PM -0700, Linus Torvalds wrote: > > > > On Thu, Oct 3, 2013 at 1:41 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > The problem is this: > > > > > A = 1, B = 1 > > > > > CPU1: > > > > > A = 0 > > > > > <full barrier> > > > > > synchronize_rcu() > > > > > read B > > > > > > > > > > CPU2: > > > > > rcu_read_lock() > > > > > B = 0 > > > > > read A > > > > /me scratches his head... > > > > OK, for CPU2 to see 1 from its read from A, the corresponding RCU > > read-side critical section must have started before CPU1 did A=0. This > > means that this same RCU read-side critical section must have started > > before CPU1's synchronize_rcu(), which means that it must complete > > before that synchronize_rcu() returns. Therefore, CPU2's B=0 must > > execute before CPU1's read of B, hence that read of B must return zero. > > > > Conversely, if CPU1's read from B returns 1, we know that CPU2's > > RCU read-side critical section must not have completed until after > > CPU1's synchronize_rcu() returned, which means that the RCU read-side > > critical section must have started after that synchronize_rcu() started, > > so CPU1's assignment to A must also have already happened. Therefore, > > CPU2's read from A must return zero. > > Yeah, that makes sense. > > I think too much time spent staring at the *implementation* of RCU and > the exciting assumptions it has to make about barriers or memory > operations leaking out of the implementations of the RCU primitives (for > instance, the fun needed to guarantee a memory barrier on all CPUs, or > to safely use non-atomic operations inside RCU itself) makes it entirely > too difficult to look at a perfectly ordinary *use* of RCU primitives > and see the obvious. :) I must confess that my first thought upon seeing Al's example was "but of course CPU2's write to B and read from A can be reordered by either the compiler or the CPU!" I had to look again myself. ;-) 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
