linux-kernel - Re: Using RCU with rcu_read

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Message-ID: <d120d5000706151325i5c84f00ft9a33175d8e3f9097@mail.gmail.com>
Date:	Fri, 15 Jun 2007 16:25:02 -0400
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	"Dipankar Sarma" <dipankar@...ibm.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Using RCU with rcu_read_lock()?

On 6/15/07, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> On Sat, Jun 16, 2007 at 12:59:40AM +0530, Dipankar Sarma wrote:
> > On Fri, Jun 15, 2007 at 09:04:19PM +0200, Peter Zijlstra wrote:
> > > On Fri, 2007-06-15 at 15:00 -0400, Dmitry Torokhov wrote:
> > > > Hi,
> > > >
> > > > I have a piece of code that is always called under a spinlock with
> > > > interrups disabled. Within that piece of code I iterate through a
> > > > list. I have another piece of code that wants to modify that list. I
> > > > have 2 options:
> > > >
> > > > I don't want to do 1) because the otheir piece of code does not really
> > > > care about object owning the spinlock and so acquiring the spinlock is
> > > > "not nice". However it is guaranteed that the piece of code that
> > > > accesses lock runs atomically with interrupts disabled. So
> > > > rcu_read_lock() would be superfluos there.
> > > >
> > > > Is it possible to still use list_for_each_rcu() and friends to access
> > > > that list without rcu_read_lock()? Or it is betteruse complete RCU
> > > > interface and eat cost of couple of extra instrctions?
> > >
> > > Yes, preemptible rcu requires that you use the full interface, also, it
> > > more clearly documents the code. Trying to find code that breaks these
> > > assumptions is very tedious work after the fact.
> > >
> > > Please do use the RCU interface in full.
> >
> > As Peter said, you should use the strict RCU APIs and not rely
> > on the current implementation of RCU to optimize. Things change.
> > Plus static/dynamic checking becomes easier that way.
>
> What they said!!!
>
> There are a couple of other options, however:
>
> 1.      Use preempt_disable() and preempt_enable() on the read side,
>        and synchronize_sched() on the update side.
>
> 2.      Use local_irq_save() and local_irq_restore() on the read side,
>        and synchronize_sched() on the update side.  Usually not
>        competitive -- unless interrupts needed to be disabled for some
>        other reason anyway.  Which you in fact say that you do.

Right. The callsite that iterates through the list is essentially
protected by spin_lock_irqsave()/spin_unlock_irqrestore() - needed for
other reasons (such as updating internal state of a device - and that
can happen from different contexts).

>
> I believe that #2 might do what you want.  But please, PLEASE carefully
> comment this usage!!!
>

Would there be a reson not to use #2 but rather full RCU with
rcu_read_lock()/synchronize_rcu()?

Thank you.

-- 
Dmitry
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