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

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Message-ID: <20080422133631.GA28594@linux.vnet.ibm.com>
Date:	Tue, 22 Apr 2008 06:36:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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 Tue, Apr 22, 2008 at 09:03:04AM +0800, Herbert Xu wrote:
> 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.

Yep, ACCESS_ONCE() is quite recent -- within the last year.  So I should
have modified the list_for_each.*rcu() macros when I made that change.

> 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.

And I still have the bug in to gcc:

	http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33102

Interesting, currently in status "unconfirmed"...  I guess I should
supply a test case.

							Thanx, Paul
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