linux-kernel - Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

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Message-ID: <20160714193800.GN30154@twins.programming.kicks-ass.net>
Date:	Thu, 14 Jul 2016 21:38:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, tj@...nel.org,
	john.stultz@...aro.org, dimitrysh@...gle.com, romlem@...gle.com,
	ccross@...gle.com, tkjos@...gle.com
Subject: Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

On Thu, Jul 14, 2016 at 12:29:59PM -0700, Paul E. McKenney wrote:
> On Thu, Jul 14, 2016 at 09:20:18PM +0200, Peter Zijlstra wrote:
> >  /**
> > + * rcu_sync_sabotage() - Sabotage a fresh rcu_sync instance
> > + * @rsp: Pointer to rcu_sync structure to be sabotaged
> > + *
> > + * Must be called after rcu_sync_init() and before first use.
> > + *
> > + * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}() pairs
> > + * turn into NO-OPs.
> > + */
> > +void rcu_sync_sabotage(struct rcu_sync *rsp)
> > +{
> > +	rsp->gp_count++;
> > +	rsp->gp_state = !GP_IDLE;
> 
> ???  A very strange way to say GP_PENDING.  A new GP_DISABLED, perhaps?

Right, so the important thing is that its not GP_IDLE, the rest doesn't
really matter.

This forces rcu_sync_is_idle() to return false. The skewed gp_count
ensures rcu_sync_{enter,exit}() pairs no-op.