Re: [PATCH RFC tip/core/rcu] SRCU rewrite

[Peter Zijlstra]

On Tue, Nov 15, 2016 at 01:54:50PM +0000, Mathieu Desnoyers wrote:
> ----- On Nov 15, 2016, at 2:51 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
> 
> > On Mon, Nov 14, 2016 at 10:36:36AM -0800, Paul E. McKenney wrote:
> >> SRCU uses two per-cpu counters: a nesting counter to count the number of
> >> active critical sections, and a sequence counter to ensure that the nesting
> >> counters don't change while they are being added together in
> >> srcu_readers_active_idx_check().
> >> 
> >> This patch instead uses per-cpu lock and unlock counters. Because the both
> >> counters only increase and srcu_readers_active_idx_check() reads the unlock
> >> counter before the lock counter, this achieves the same end without having
> >> to increment two different counters in srcu_read_lock(). This also saves a
> >> smp_mb() in srcu_readers_active_idx_check().
> > 
> > A very small improvement... I feel SRCU has much bigger issues :/
> 
> Do you have specific issues in mind ?

The smp_mb()s in read_{un,}lock() and the lock in call_srcu() come to
mind.