Date: Wed, 11 Jan 2006 22:51:15 -0800
From: "Paul E. McKenney" <paulmck@us.ibm.com>
To: Keith Owens <kaos@sgi.com>
Cc: John Hesterberg <jh@sgi.com>, Matt Helsley <matthltc@us.ibm.com>,
       Jes Sorensen <jes@trained-monkey.org>,
       Shailabh Nagar <nagar@watson.ibm.com>, Andrew Morton <akpm@osdl.org>,
       Jay Lan <jlan@engr.sgi.com>, LKML <linux-kernel@vger.kernel.org>,
       elsa-devel@lists.sourceforge.net, lse-tech@lists.sourceforge.net,
       CKRM-Tech <ckrm-tech@lists.sourceforge.net>, Paul Jackson <pj@sgi.com>,
       Erik Jacobson <erikj@sgi.com>, Jack Steiner <steiner@sgi.com>
Subject: Re: [Lse-tech] Re: [ckrm-tech] Re: [PATCH 00/01] Move Exit Connectors
Message-ID: <20060112065115.GB23673@us.ibm.com>
Reply-To: paulmck@us.ibm.com
References: <20060112050453.GA23673@us.ibm.com> <15187.1137046741@kao2.melbourne.sgi.com>
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On Thu, Jan 12, 2006 at 05:19:01PM +1100, Keith Owens wrote:
> "Paul E. McKenney" (on Wed, 11 Jan 2006 21:04:53 -0800) wrote:
> >On Thu, Jan 12, 2006 at 02:29:52PM +1100, Keith Owens wrote:
> >> John Hesterberg (on Wed, 11 Jan 2006 15:39:10 -0600) wrote:
> >> >On Wed, Jan 11, 2006 at 01:02:10PM -0800, Matt Helsley wrote:
> >> >> 	Have you looked at Alan Stern's notifier chain fix patch? Could that be
> >> >> used in task_notify?
> >> >
> >> >I have two concerns about an all-tasks notification interface.
> >> >First, we want this to scale, so don't want more global locks.
> >> >One unique part of the task notify is that it doesn't use locks.
> >> 
> >> Neither does Alan Stern's atomic notifier chain.  Indeed it cannot use
> >> locks, because the atomic notifier chains can be called from anywhere,
> >> including non maskable interrupts.  The downside is that Alan's atomic
> >> notifier chains require RCU.
> >> 
> >> An alternative patch that requires no locks and does not even require
> >> RCU is in http://marc.theaimsgroup.com/?l=linux-kernel&m=113392370322545&w=2
> >
> >Interesting!  Missed this on the first time around...
> >
> >But doesn't notifier_call_chain_lockfree() need to either disable
> >preemption or use atomic operations to update notifier_chain_lockfree_inuse[]
> >in order to avoid problems with preemption?
> 
> OK, I have thought about it and ...
> 
>   notifier_call_chain_lockfree() must be called with preempt disabled.
> 
> The justification for this routine is to handle all the nasty
> corner cases in the notify_die() and similar chains, including panic,
> spinlocks being held and even non maskable interrupts.  It is silly to
> try to make notifier_call_chain_lockfree() handle the preemptible case
> as well.

Fair enough!  A comment, perhaps?  In a former life I would have also
demanded debug code to verify that preemption/interrupts/whatever were
actually disabled, given the very subtle nature of any resulting bugs...

> If notifier_call_chain_lockfree() is to be called for task
> notification, then the caller must disable preempt around the call to
> notifier_call_chain_lockfree().  Scalability over lots of cpus should
> not be an issue, especially if notifier_chain_lockfree_inuse[] is
> converted to a per cpu variable.  The amount of time spent with preempt
> disabled is proportional to the number of registered callbacks on the
> task notifcation chain and to the amount of work performed by those
> callbacks, neither of which should be high.

Ah, but the guys doing the latency measurements will be the judge of
that, right?  ;-)

						Thanx, Paul
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