linux-kernel - Re: [PATCH v3 0/2] Use blocked_lock_lock only to protect blocked

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Date:	Sat, 7 Mar 2015 09:09:27 -0500
From:	Jeff Layton <jlayton@...chiereds.net>
To:	Daniel Wagner <daniel.wagner@...-carit.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH v3 0/2] Use blocked_lock_lock only to protect
 blocked_hash

On Sat, 7 Mar 2015 09:00:41 -0500
Jeff Layton <jlayton@...chiereds.net> wrote:

> On Fri,  6 Mar 2015 08:53:30 +0100
> Daniel Wagner <daniel.wagner@...-carit.de> wrote:
> 
> > Hi,
> > 
> > Finally, I got a bigger machine and did a quick test round. I expected
> > to see some improvements but the resutls do not show any real gain. So
> > they are merely refactoring patches.
> > 
> 
> Ok, in that case is there any point in merging these? I'm all for
> breaking up global locks when it makes sense, but if you can't
> demonstrate a clear benefit then I'm less inclined to take the churn.
> 
> Perhaps we should wait to see if a benefit emerges when/if you convert
> the lglock code to use normal spinlocks (like Andi suggested)? That
> seems like a rather simple conversion, and I don't think it's
> "cheating" in any sense of the word.
> 
> I do however wonder why Nick used arch_spinlock_t there when he wrote
> the lglock code instead of normal spinlocks. Was it simply memory usage
> considerations or something else?
> 

Hmm...to answer my own question. The (old) LWN article here seems to
suggest that he did it that way to avoid preemption:

    http://lwn.net/Articles/401738/

I don't think we need to avoid being preempted in the file-locking
code, but I'm not sure about stop_machine.c. Is that necessary there?
The comment in queue_stop_cpus_work seems to indicate that it may be.

-- 
Jeff Layton <jlayton@...chiereds.net>
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