linux-kernel - Re: Using RCU with rcu_read

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Message-ID: <20070615192940.GA2996@in.ibm.com>
Date:	Sat, 16 Jun 2007 00:59:40 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Using RCU with rcu_read_lock()?

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.

Thanks
Dipankar
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