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

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Date:	Sun, 7 Dec 2008 09:28:54 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	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>, "Theodore Ts'o" <tytso@....edu>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Sun, 07 Dec 2008 14:28:00 +0100 Eric Dumazet <dada1@...mosbay.com> wrote:

> Andrew Morton a __crit :
> > On Wed, 03 Dec 2008 21:24:36 +0100 Eric Dumazet <dada1@...mosbay.com> wrote:
> > 
> >> Eric Dumazet a __crit :
> >>
> >> 1) __percpu_counter_sum() is buggy, it should not write
> >> on per_cpu_ptr(fbc->counters, cpu), or another cpu
> >> could get its changes lost.
> >>
> >> __percpu_counter_sum should be read only (const struct percpu_counter *fbc),
> >> and no locking needed.
> > 
> > No, we can't do this - it will break ext4.
> > 
> > Take a closer look at 1f7c14c62ce63805f9574664a6c6de3633d4a354 and at
> > e8ced39d5e8911c662d4d69a342b9d053eaaac4e.
> > 
> > I suggest that what we do is to revert both those changes.  We can
> > worry about the possibly-unneeded spin_lock later, in a separate patch.
> > 
> > It should have been a separate patch anyway.  It's conceptually
> > unrelated and is not a bugfix, but it was mixed in with a bugfix.
> > 
> > Mingming, this needs urgent consideration, please.  Note that I had to
> > make additional changes to ext4 due to the subsequent introduction of
> > the dirty_blocks counter.
> > 
> > 
> > Please read the below changelogs carefully and check that I have got my
> > head around this correctly - I may not have done.
> > 
> 
> 
> Hum... e8ced39d5e8911c662d4d69a342b9d053eaaac4e is probably following
> the wrong path, but I see the intent. Even in the 'nr_files' case, it could
> help to reduce excessive calls to percpu_counter_sum()
> 

We should fix this in 2.6.28 - right now percpu_counter_sum() is subtly
corrupting the counter's value.

I sent two revert patches which I hope to merge into 2.6.28.  Could you
guys please read/review/maybe-test them?

They will make ext4 as slow as it was in 2.6.26, but presumably that's
not a catastrophe.

> What we can do is to use two s64 counters (only in SMP):

We can do lots of things in 2.6.29.  Including just making ->counters
an array of atomic_t.

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