Re: [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock_sched

[Mikulas Patocka]

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 <mpatocka@xxxxxxxxxx>
> 
> 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

Mikulas
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