Re: Using RCU with rcu_read_lock()?

From: Dmitry Torokhov
Date: Fri Jun 15 2007 - 16:25:18 EST

On 6/15/07, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> 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

Thank you.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at