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[Xen-devel] Re: rcu support in xen

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: [Xen-devel] Re: rcu support in xen
From: "Mike D. Day" <ncmike@xxxxxxxxxx>
Date: Sun, 25 Mar 2007 10:12:40 -0400
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On 25/03/07 09:28 +0100, Keir Fraser wrote:
On 24/3/07 19:22, "Mike D. Day" <ncmike@xxxxxxxxxx> wrote:

Due to recent conversion of get_domain_by_id to use rcu techniques I
looked at the file xen/include/xen/rcupdate.h and noticed
rcu_read_lock is defined as:

define rcu_read_lock(x)       do { } while (0)

as well as the corresponding unlock and other related macros.

Apologies if this has already been answered, but when will the real
rcu code get into -unstable?

The whole point of RCU is that read-side critical sections have no (or very
little) synchronisation overhead. In Xen the rcu_read_[un]lock functions are
only present to make the code clearer and to seome extent self documenting.
On Linux they also disable involuntary preemption during the critical
section (which is not needed on Xen, of course).

So, in short, Xen already has a complete RCU implementation.

I see, the thing that threw me off was the sparse checker code in the
linux rcu_read_lock() implementation, as a comparison. However, it
doesn't do anything either.

By the way, the linux implementation does not require a dummy lock

#define rcu_read_lock() \
        do { \
                preempt_disable(); \
                __acquire(RCU); \
        } while(0)

This is better - in my opinion, it makes it clearer that the rcu reader
is not actually synchronizing on a spinlock.
I'd be happy to submit a patch for the reader lock/unlock macros.


Mike D. Day
Cell: 919 412-3900
Sametime: ncmike@xxxxxxxxxx AIM: ncmikeday  Yahoo: ultra.runner
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