linux-kernel - Re: [PATCH 2/2] ipc: semaphores: do not hold ipc lock more than necessary

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Message-ID: <1362259114.3602.6.camel@buesod1.americas.hpqcorp.net>
Date:	Sat, 02 Mar 2013 13:18:34 -0800
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	"Low, Jason" <jason.low2@...com>,
	linux-tip-commits@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aquini@...hat.com,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Larry Woodman <lwoodman@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] ipc: semaphores: do not hold ipc lock more than
 necessary

On Fri, 2013-03-01 at 17:20 -0800, Linus Torvalds wrote:
> On Fri, Mar 1, 2013 at 4:16 PM, Davidlohr Bueso <davidlohr.bueso@...com> wrote:
> > +static inline struct sem_array *sem_obtain_object(struct ipc_namespace *ns, int id)
> > +{
> > +       struct kern_ipc_perm *ipcp = ipc_obtain_object(&sem_ids(ns), id);
> > +
> > +       if (IS_ERR(ipcp))
> > +               return (struct sem_array *)ipcp;
> 
> This should use ERR_CAST() to make it more obvious what's going on.
> 
> > +static inline struct sem_array *sem_obtain_object_check(struct ipc_namespace *ns,
> > +                                                       int id)
> > +{
> > +       struct kern_ipc_perm *ipcp = ipc_obtain_object_check(&sem_ids(ns), id);
> > +
> > +       if (IS_ERR(ipcp))
> > +               return (struct sem_array *)ipcp;
> 
> Same here.

Ok

> 
> > +/*
> > + * Call inside the rcu read section.
> > + */
> > +static inline void sem_getref(struct sem_array *sma)
> > +{
> > +       spin_lock(&(sma)->sem_perm.lock);
> > +       ipc_rcu_getref(sma);
> > +       ipc_unlock(&(sma)->sem_perm);
> > +}
> 
> This really makes me wonder if we shouldn't just use an atomic counter
> for "refcount". But I guess that would be a separate patch.
> 

Ah, yes indeed.

> But all the uses of refcount really look like the normal atomic ops
> migth be the right thing. Especially if we no longer expect to hold
> the lock most of the time.
> 
> > +               spin_lock(&sma->sem_perm.lock);
> 
> I really would almost want to make these things be "ipc_lock_object()"
> rather than an open-coded spinlock like this. But that's not a big
> deal.

Sure.

> 
> Patch looks fine to me in general.
> 

Thanks for taking a look!

Davidlohr

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