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