On Thu, Oct 25, 2012 at 10:54:11AM -0400, Mikulas Patocka wrote: > > > On Wed, 24 Oct 2012, Paul E. McKenney wrote: > > > On Mon, Oct 22, 2012 at 07:39:16PM -0400, Mikulas Patocka wrote: > > > Use rcu_read_lock_sched / rcu_read_unlock_sched / synchronize_sched > > > instead of rcu_read_lock / rcu_read_unlock / synchronize_rcu. > > > > > > This is an optimization. The RCU-protected region is very small, so > > > there will be no latency problems if we disable preempt in this region. > > > > > > So we use rcu_read_lock_sched / rcu_read_unlock_sched that translates > > > to preempt_disable / preempt_disable. It is smaller (and supposedly > > > faster) than preemptible rcu_read_lock / rcu_read_unlock. > > > > > > Signed-off-by: Mikulas Patocka <mpato...@redhat.com> > > > > OK, as promised/threatened, I finally got a chance to take a closer look. > > > > The light_mb() and heavy_mb() definitions aren't doing much for me, > > the code would be cleared with them expanded inline. And while the > > approach of pairing barrier() with synchronize_sched() is interesting, > > it would be simpler to rely on RCU's properties. The key point is that > > if RCU cannot prove that a given RCU-sched read-side critical section > > is seen by all CPUs to have started after a given synchronize_sched(), > > then that synchronize_sched() must wait for that RCU-sched read-side > > critical section to complete. > > Also note that you can define both light_mb() and heavy_mb() to be > smp_mb() and slow down the reader path a bit and speed up the writer path. > > On architectures with in-order memory access (and thus smp_mb() equals > barrier()), it doesn't hurt the reader but helps the writer, for example: > #ifdef ARCH_HAS_INORDER_MEMORY_ACCESS > #define light_mb() smp_mb() > #define heavy_mb() smp_mb() > #else > #define light_mb() barrier() > #define heavy_mb() synchronize_sched() > #endif
Except that there are no systems running Linux with in-order memory access. Even x86 and s390 require a barrier instruction for smp_mb() on SMP=y builds. Thanx, Paul -- 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/