linux-kernel - Re: [PATCH 3/3] serial: 8250: Add a wakeup

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Message-ID: <20120123164511.GE2434@linux.vnet.ibm.com>
Date:	Mon, 23 Jan 2012 08:45:11 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Simon Glass <sjg@...omium.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH 3/3] serial: 8250: Add a wakeup_capable module param

On Sat, Jan 21, 2012 at 12:49:35AM +0100, Rafael J. Wysocki wrote:
> On Friday, January 20, 2012, Paul E. McKenney wrote:
> > On Fri, Jan 20, 2012 at 01:03:34AM +0100, Rafael J. Wysocki wrote:
> > > On Thursday, January 19, 2012, Paul E. McKenney wrote:
> > > > On Thu, Jan 19, 2012 at 01:02:58AM +0100, Rafael J. Wysocki wrote:
> > > > > On Wednesday, January 18, 2012, Paul E. McKenney wrote:
> > > > > > On Wed, Jan 18, 2012 at 02:15:59PM -0800, Simon Glass wrote:
> > > [...] 
> > > > > Yes, you can, but then I'd say it's not necessary for user space to
> > > > > be able to carry that out in a tight loop.  So, it seems, alternatively,
> > > > > we could make that loop a bit less tight, e.g. by adding an arbitrary
> > > > > sleep to the user space interface for the "disable" case.
> > > > 
> > > > Good point, that would work just as well and be simpler.
> > > 
> > > Thanks for the confirmation! :-)
> > > 
> > > By the way, I wonder, would it help to add synchronize_rcu() to
> > > wakeup_source_add() too?  Then, even if device_wakeup_enable() and
> > > device_wakeup_disable() are executed in a tight loop for the same
> > > device, the list_add/list_del operations will always happen in
> > > different RCU cycles (or at least it seems so).
> > 
> > I cannot immediately see how adding a synchronize_rcu() to
> > wakeup_source_add() would help anything.  You only need to wait for a
> > grace period on removal, not (normally) on addition.  The single grace
> > period during removal will catch up all other asynchronous RCU grace
> > period requests on that CPU.
> > 
> > Or am I missing your point?
> 
> Well, I was thinking about the failure scenario you mentioned where
> executing enable/disable in a tight loop might exhaust system memory
> (if I understood it correctly).

Ah, got it.  If they are executing this in a tight loop, there will be
little difference between doing one synchronize_rcu() per pass through
the loop or doing two.  So we should be just fine with the single instance
of synchronize_rcu() per loop.

							Thanx, Paul

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