linux-ext4 - Re: [PATCH] percpu_counter: Fix __percpu_counter

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Message-ID: <20081208221241.GA2501@mit.edu>
Date:	Mon, 8 Dec 2008 17:12:41 -0500
From:	Theodore Tso <tytso@....edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Sun, Dec 07, 2008 at 08:52:50PM -0800, Andrew Morton wrote:
> 
> The first patch which was added (pre-2.6.27) was "percpu_counter: new
> function percpu_counter_sum_and_set".  This added the broken-by-design
> percpu_counter_sum_and_set() function, **and used it in ext4**.
> 

Mea culpa, I was the one who reviewed Mingming's patch, and missed
this.  Part of the problem was that percpu_counter.c isn't well
documented, and I so saw the spinlock, but didn't realize it only
protected reference counter, and not the per-cpu array.  I should have
read through code more thoroughly before approving the patch.

I suppose if we wanted we could add a rw spinlock which mediates
access to a "foreign" cpu counter (i.e., percpu_counter_add gets a
shared lock, and percpu_counter_set needs an exclusive lock) but it's
probably not worth it.

Actually, if all popular architectures had a hardware-implemented
atomic_t, I wonder how much ext4 really needs the percpu counter,
especially given ext4's multiblock allocator; with ext3, given that
each block allocation required taking a per-filesystem spin lock,
optimizing away that spinlock was far more important for improving
ext3's scalability.  But with the multiblock allocator, it may that
we're going through a lot more effort than what is truly necessary.

						- Ted
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