linux-kernel - Re: [PATCH RFC 01/15] uaccess: count pagefault_disable() levels in pagefault

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Message-ID: <20150507111239.GB15284@gmail.com>
Date:	Thu, 7 May 2015 13:12:39 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...hat.com,
	yang.shi@...driver.com, bigeasy@...utronix.de,
	benh@...nel.crashing.org, paulus@...ba.org,
	akpm@...ux-foundation.org, heiko.carstens@...ibm.com,
	schwidefsky@...ibm.com, borntraeger@...ibm.com, mst@...hat.com,
	tglx@...utronix.de, David.Laight@...LAB.COM, hughd@...gle.com,
	hocko@...e.cz, ralf@...ux-mips.org, herbert@...dor.apana.org.au,
	linux@....linux.org.uk, airlied@...ux.ie, daniel.vetter@...el.com,
	linux-mm@...ck.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RFC 01/15] uaccess: count pagefault_disable() levels in
 pagefault_disabled


* David Hildenbrand <dahi@...ux.vnet.ibm.com> wrote:

> > AFAICR we did this to avoid having to do both:
> > 
> > 	preempt_disable();
> > 	pagefault_disable();
> > 
> > in a fair number of places -- just like this patch-set does, this is
> > touching two cachelines where one would have been enough.
> > 
> > Also, removing in_atomic() from fault handlers like you did
> > significantly changes semantics for interrupts (soft, hard and NMI).
> > 
> > So while I agree with most of these patches, I'm very hesitant on the
> > above little detail.
> 
> Just to make sure we have a common understanding (as written in my 
> cover letter):
> 
> Your suggestion won't work with !CONFIG_PREEMPT 
> (!CONFIG_PREEMPT_COUNT). If there is no preempt counter, in_atomic() 
> won't work. So doing a preempt_disable() instead of a 
> pagefault_disable() is not going to work. (not sure how -RT handles 
> that - most probably with CONFIG_PREEMPT_COUNT being enabled, due to 
> atomic debug).
> 
> That's why I dropped that check for a reason.

So, what's the point of disabling the preempt counter?

Looks like the much simpler (and faster) solution would be to 
eliminate CONFIG_PREEMPT_COUNT (i.e. make it always available), and 
use it for pagefault-disable.

> This patchset is about decoupling both concept. (not ending up with 
> to mechanisms doing almost the same)

So that's really backwards: just because we might not have a handy 
counter we introduce _another one_, and duplicate checks for it ;-)

Why not keep a single counter, if indeed what we care about most in 
the pagefault_disable() case is atomicity?

Thanks,

	Ingo
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