Re: [PATCH tip/core/rcu 06/23] rcu: Break up rcu_gp_kthread() into subfunctions

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, mingo@elte.hu,
	laijs@cn.fujitsu.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
	niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org,
	Valdis.Kletnieks@vt.edu, dhowells@redhat.com,
	eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com,
	sbw@mit.edu, patches@linaro.org
Subject: Re: [PATCH tip/core/rcu 06/23] rcu: Break up rcu_gp_kthread() into subfunctions
Date: Thu, 6 Sep 2012 13:30:31 -0700	[thread overview]
Message-ID: <20120906203031.GS2448@linux.vnet.ibm.com> (raw)
In-Reply-To: <20120906184921.GA5738@jtriplet-mobl1>

On Thu, Sep 06, 2012 at 11:49:21AM -0700, Josh Triplett wrote:
> On Thu, Sep 06, 2012 at 10:32:07AM -0700, Paul E. McKenney wrote:
> > On Thu, Sep 06, 2012 at 03:39:51PM +0200, Peter Zijlstra wrote:
> > > On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
> > > > +static int rcu_gp_kthread(void *arg)
> > > > +{
> > > > +       struct rcu_state *rsp = arg;
> > > > +       struct rcu_node *rnp = rcu_get_root(rsp);
> > > > +
> > > > +       for (;;) {
> > > > +
> > > > +               /* Handle grace-period start. */
> > > > +               for (;;) {
> > > > +                       wait_event_interruptible(rsp->gp_wq, rsp->gp_flags);
> > > > +                       if (rsp->gp_flags && rcu_gp_init(rsp))
> > > > +                               break;
> > > > +                       cond_resched();
> > > > +                       flush_signals(current);
> > > > +               }
> > > >  
> > > >                 /* Handle grace-period end. */
> > > >                 for (;;) {
> > > >                         wait_event_interruptible(rsp->gp_wq,
> > > >                                                  !ACCESS_ONCE(rnp->qsmask) &&
> > > >                                                  !rcu_preempt_blocked_readers_cgp(rnp));
> > > >                         if (!ACCESS_ONCE(rnp->qsmask) &&
> > > > +                           !rcu_preempt_blocked_readers_cgp(rnp) &&
> > > > +                           rcu_gp_cleanup(rsp))
> > > >                                 break;
> > > > +                       cond_resched();
> > > >                         flush_signals(current);
> > > >                 }
> > > >         }
> > > >         return 0;
> > > >  } 
> > > 
> > > Should there not be a kthread_stop() / kthread_park() call somewhere in
> > > there?
> > 
> > The kthread stops only when the system goes down, so no need for any
> > kthread_stop() or kthread_park().  The "return 0" suppresses complaints
> > about falling of the end of a non-void function.
> 
> Huh, I thought GCC knew to not emit that warning unless it actually
> found control flow reaching the end of the function; since the infinite
> loop has no break in it, you shouldn't need the return.  Annoying.
> 
> > > Also, it could be me, but all those nested for (;;) loops make the flow
> > > rather non-obvious.
> > 
> > For those two loops, I suppose I could pull the cond_resched() and
> > flush_signals() to the top, and make a do-while out of it.
> 
> I think it makes more sense to move the wait_event_interruptible to the
> bottom, and make a while out of it.

I know!!!  Let's compromise and put the loop exit in the middle of the
loop!!!  Oh, wait...

							;-), Paul