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From: Blaisorblade <blaisorblade@yahoo.it>
To: user-mode-linux-devel@lists.sourceforge.net
Subject: Re: [uml-devel] Re: [PATCH 07/10] uml: avoid fixing faults while atomic
Date: Wed, 21 Sep 2005 22:22:50 +0200
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Cc: Andrew Morton <akpm@osdl.org>, torvalds@osdl.org, jdike@addtoit.com,
       linux-kernel@vger.kernel.org
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On Wednesday 21 September 2005 21:49, Andrew Morton wrote:
> "Paolo 'Blaisorblade' Giarrusso" <blaisorblade@yahoo.it> wrote:
> > From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

> The in_atomic() test in x86's do_page_fault() is in fact a message passed
> into it from filemap.c's kmap_atomic().
Ok, this can be ok, but:
> It has accidental side-effects, 
> such as making copy_to_user() fail if inside spinlocks when
> CONFIG_PREEMPT=y.
Sorry, but should it ever succeed inside spinlocks? I mean, should it ever 
call down() inside spinlocks? (We never do down_trylock, and ever if we did 
the x86 trick, that wouldn't make the whole thing safe at all - they still 
take the spinlock and potentially sleep. And it's legal only if no spinlock 
is held).

Even if spinlocks don't always trigger in_atomic() - which means that we'd 
need to have a better fix for this.

(Btw, I took the above reasoning from something said, as an aside, on LWN.net 
kernel page, about the FUTEX deadlock on mm->mmap_sem of ~ 2.6.8 - yes, it 
wasn't the full truth, but not totally dumb).

> So I think this change is only needed if UML implements kmap_atomic, as in
> arch/i386/mm/highmem.c, which it surely does not do?
NACK, see above.

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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