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

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Message-ID: <20161115135912.GJ3142@twins.programming.kicks-ass.net>
Date:   Tue, 15 Nov 2016 14:59:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        dvhart <dvhart@...ux.intel.com>, fweisbec <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        bobby prani <bobby.prani@...il.com>, ldr709 <ldr709@...il.com>
Subject: Re: [PATCH RFC tip/core/rcu] SRCU rewrite

On Tue, Nov 15, 2016 at 01:54:50PM +0000, Mathieu Desnoyers wrote:
> ----- On Nov 15, 2016, at 2:51 AM, Peter Zijlstra peterz@...radead.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.