linux-kernel - Re: [RFC PATCH 02/12] On Tue, 23 Sep 2008, David Miller wrote:

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Message-ID: <alpine.LFD.2.00.0810050959420.3398@apollo>
Date:	Sun, 5 Oct 2008 10:51:45 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jesse Brandeburg <jesse.brandeburg@...il.com>
cc:	Jiri Kosina <jkosina@...e.cz>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	David Miller <davem@...emloft.net>, jesse.brandeburg@...el.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	kkeil@...e.de, agospoda@...hat.com, arjan@...ux.intel.com,
	david.graham@...el.com, bruce.w.allan@...el.com,
	john.ronciak@...el.com, chris.jones@...onical.com,
	tim.gardner@...onical.com, airlied@...il.com,
	Olaf Kirch <okir@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 02/12] On Tue, 23 Sep 2008, David Miller wrote:

On Sat, 4 Oct 2008, Jesse Brandeburg wrote:
> > Exactly. The access to a ro region results in a fault. I have nowhere
> > seen that trigger, but I can reproduce the trylock() WARN_ON, which
> > confirms that there is concurrent access to the NVRAM registers. The
> > backtrace pattern is similar to the one you have seen.
> 
> are you still getting WARN_ON *with* all the mutex based fixes already applied?

The WARN_ON triggers with current mainline. Is there any fixlet in
Linus tree missing ?

> with the mutex patches in place (without protection patch) we are
> still reproducing the issue, until we apply the set_memory_ro patch.

That does not make sense to me. If the memory_ro patch is providing
_real_ protection then you _must_ run into an access violation. If not,
then the patch just papers over the real problem in some mysterious
way.

The patch does:

+	set_memory_rw((unsigned long)hw->flash_address,
+	              hw->flash_len >> PAGE_SHIFT);
 	writew(val, hw->flash_address + reg);
+	set_memory_ro((unsigned long)hw->flash_address,
+	              hw->flash_len >> PAGE_SHIFT);

This changes massively the timing of the flash access. Could this be
the problem on the machine which needs the set_memory_ro patch to
survive ?

> I had no luck on friday setting a hardware breakpoint on memory access
> with kgdb to catch the writer with a breakpoint.

Well, why should you get a hardware breakpoint when the _ro protection
does not trigger in the first place ?

Granted there could be a _rw alias mapping, but then the problem must
be still visible with the _ro patch applied.

Thanks,

	tglx

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