Date: Tue, 15 Nov 2016 14:59:12 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Lai Jiangshan <jiangshanlai@gmail.com>,
        dipankar <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>, rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>, dvhart <dvhart@linux.intel.com>,
        fweisbec <fweisbec@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
        bobby prani <bobby.prani@gmail.com>, ldr709 <ldr709@gmail.com>
Subject: Re: [PATCH RFC tip/core/rcu] SRCU rewrite
Message-ID: <20161115135912.GJ3142@twins.programming.kicks-ass.net>
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On Tue, Nov 15, 2016 at 01:54:50PM +0000, Mathieu Desnoyers wrote:
> ----- On Nov 15, 2016, at 2:51 AM, Peter Zijlstra peterz@infradead.org 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.