linux-ext4 - Re: 2.6.25-git2: BUG: unable to handle kernel paging request at ffffffffffffffff

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Date:	Tue, 22 Apr 2008 09:03:04 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org
Subject: Re: 2.6.25-git2: BUG: unable to handle kernel paging request at ffffffffffffffff

On Mon, Apr 21, 2008 at 08:49:58AM -0700, Linus Torvalds wrote:
>
> That is *not* the main problem.
> 
> If you use "rcu_dereference()" on the wrong access, it not only loses the 
> "smp_read_barrier_depends()" (which is a no-op on all sane architectures 
> anyway), but it loses the ACCESS_ONCE() thing *entirely*.

Actually rcu_dereference didn't have ACCESS_ONCE when I did this.
That only appearaed later with the preemptible RCU work.

The original purpose of rcu_dereference was exactly to replace the
explicit barriers that people were using for RCU, nothing more,
nothing less.

Oh and I totally agree that the compiler is going to generate insane
code whenever ACCESS_ONCE is used.  In this case we may have avoided
it by rearranging the code, but in general the introduction of ACCESS_ONCE
in rcu_dereference is likely to have a negative impact on the code
generated.

Remember that "volatile" discussion? I think this is where it all came
from.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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