Date: Wed, 1 Jul 2015 15:48:06 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via
 expedited GPs
Message-ID: <20150701134806.GF25159@twins.programming.kicks-ass.net>
References: <20150630214805.GA7795@linux.vnet.ibm.com>
 <1435700910-9104-1-git-send-email-paulmck@linux.vnet.ibm.com>
 <1435700910-9104-2-git-send-email-paulmck@linux.vnet.ibm.com>
 <20150701100521.GP19282@twins.programming.kicks-ass.net>
 <20150701134143.GY3717@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150701134143.GY3717@linux.vnet.ibm.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1218
Lines: 28

On Wed, Jul 01, 2015 at 06:41:44AM -0700, Paul E. McKenney wrote:
> On Wed, Jul 01, 2015 at 12:05:21PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 30, 2015 at 02:48:27PM -0700, Paul E. McKenney wrote:
> > >  			rsp->gp_state = RCU_GP_WAIT_FQS;
> > >  			ret = wait_event_interruptible_timeout(rsp->gp_wq,
> > > +				((gf = READ_ONCE(rsp->gp_flags)) &
> > > +				 RCU_GP_FLAG_FQS) ||
> > > +				(!READ_ONCE(rnp->qsmask) &&
> > > +				 !rcu_preempt_blocked_readers_cgp(rnp)) ||
> > > +				rcu_exp_gp_seq_done(rsp->exp_rsp,
> > > +						    rsp->gp_exp_snap),
> > > +				j);
> > >  			rsp->gp_state = RCU_GP_DONE_FQS;
> > 
> > How can the GP be done if we timed out or got interrupted?
> 
> If all the CPUs still blocking the grace period went idle, or in a
> NO_HZ_FULL kernel, entered nohz_full userspace execution.  Or, if
> certain low-probability races happen, went offline.

But what if none of those are true and we still timed out? You
unconditionally grant the GP.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/