From: Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH] percpu_counter: use local_t and atomic_long_t if possible
Date: Thu, 25 Dec 2008 23:56:58 +1030
Message-ID: <200812252356.59595.rusty@rustcorp.com.au>
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Cc: Eric Dumazet <dada1@cosmosbay.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Theodore Tso <tytso@mit.edu>,
	linux kernel <linux-kernel@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Mingming Cao <cmm@us.ibm.com>, linux-ext4@vger.kernel.org,
	Christoph Lameter <cl@linux-foundation.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
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On Tuesday 23 December 2008 22:13:21 Peter Zijlstra wrote:
> One think that sprung to mind,..
> 
> IFF we're going to implement percpu_counter with local_t and make
> local_t this funny tri-counter thing which has its own error, you need
> to fix up bdi_stat_error() - it expects to be an upper bound for the
> counter error, getting that wrong _will_ cause deadlocks.

It's OK: local_add_return() can't have error; that's why the trival
implementation disables interrupts around it.

Hope that helps,
Rusty.