From: "Paul E. McKenney" <paulmck@us.ibm.com>
To: Rusty Russell <rusty@au1.ibm.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [DOCUMENTATION] Revised Unreliable Kernel Locking Guide
Date: Mon, 15 Dec 2003 14:22:14 -0800 [thread overview]
Message-ID: <20031215222213.GA1270@us.ibm.com> (raw)
In-Reply-To: <20031215060851.1882C189E1@ozlabs.au.ibm.com>
On Mon, Dec 15, 2003 at 04:17:47PM +1100, Rusty Russell wrote:
> In message <20031212193559.GA1614@us.ibm.com> you write:
>
> > o Software Interrupt / softirq: formatting botch "of'software".
> > This would be "o'software", right?
>
> Looks ok here:
> Tasklets and softirqs both fall into the category of 'software interrupts'.
It was late and my eyes were tired. Can't even blame it on
my browser!!!
> > o Preemption: Would it be worth changing the first bit
> > of the second sentence to read something like: "In 2.5.4
> > and later, when CONFIG_PREEMPT is set, this changes:"?
>
> I was trying to make it a little future-proof: I think CONFIG_PREEMPT
> should go away some day.
I hear you! But I suspect that it may take quite some time for that day
to arrive.
> > Overzealous Prevention Of Deadlocks: Cute!!!
>
> This is untouched from the old version of the document. I had a
> troubled youth...
;-) Well, don't wring -all- of the fun out of the document!
> > Avoiding Locks: Read Copy Update
> >
> > o Might be worth noting explicitly early on that updaters are
> > running concurrently with readers. Should be obvious given
> > that the readers aren't doing any explicit synchronization,
> > but I have run into confusion on this point surprisingly often.
>
> OK. Changed the second paragraph from:
>
> How do we get rid of read locks? That is actually quite simple:
>
> to:
>
> How do we get rid of read locks? Getting rid of read locks
> means that writers may be changing the list underneath the readers.
> That is actually quite simple:
Looks good! Upon rereading... Does "wmb()" want to be "smp_wmb()"?
> > o Please add a note to the list_for_each_entry_rcu() description
> > saying that writers (who hold appropriate locks) need not use
> > the _rcu() variant.
>
> OK:
>
> Once again, there is a
> <function>list_for_each_entry_rcu()</function>
> (<filename>include/linux/list.h</filename>) to help you. Of
> course, writers can just use
> <function>list_for_each_entry()</function>, since there cannot
> be two simultaneous writers.
Also looks good!
Again, upon rereading, "read Read Copy Update code" probably wants to
be "real Read Copy Update code". I moused it this time, given
my past record with eyeballing. ;-)
> > o If nothing blocks between the call to __cache_find() and the
> > eventual object_put(), it is worthwhile to avoid the
> > reference-count manipulation. This would make all of
> > cache_find() be almost as fast as UP, rather than just
> > __cache_find().
>
> Good point. Text added at the bottom of that section:
>
> <para>
> There is a furthur optimization possible here: remember our original
> cache code, where there were no reference counts and the caller simply
> held the lock whenever using the object? This is still possible: if
> you hold the lock, noone can delete the object, so you don't need to
> get and put the reference count.
> </para>
>
> <para>
> Now, because the 'read lock' in RCU is simply disabling preemption, a
> caller which always preemption disabled between calling
disables preemption
> <function>cache_find()</function> and
> <function>object_put()</function> does not need to actually get and
> put the reference count: we could expose
> <function>__cache_find()</function> by making it non-static, and
> such callers could simply call that.
> </para>
> <para>
> The benefit here is that the reference count is not written to: the
> object is not altered in any way, which is much faster on SMP
> machines due to caching.
> </para>
Other than the grammar nit above, looks good!
> I've uploaded a new draft with these and other fixes...
Good stuff, thank you!!!
Thanx, Paul
next prev parent reply other threads:[~2003-12-16 5:27 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-12-12 5:24 [DOCUMENTATION] Revised Unreliable Kernel Locking Guide Rusty Russell
2003-12-12 15:44 ` Dave Jones
2003-12-12 16:25 ` Keith Owens
2003-12-12 18:25 ` Dave Jones
2003-12-13 0:28 ` Keith Owens
2003-12-12 21:05 ` Rob Love
2003-12-15 2:28 ` Rusty Russell
2003-12-12 19:35 ` Paul E. McKenney
2003-12-13 3:16 ` Zwane Mwaikambo
2003-12-15 5:17 ` Rusty Russell
2003-12-15 5:17 ` Rusty Russell
2003-12-15 22:22 ` Paul E. McKenney [this message]
2003-12-16 6:32 ` Rusty Russell
2003-12-13 3:15 Manfred Spraul
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