linux-kernel - Re: [PATCH v2 10/10] kernel: might_fault does not imply might

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Message-ID: <20130521111818.GI26912@twins.programming.kicks-ass.net>
Date:	Tue, 21 May 2013 13:18:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	David Howells <dhowells@...hat.com>,
	Hirokazu Takata <takata@...ux-m32r.org>,
	Michal Simek <monstr@...str.eu>,
	Koichi Yasutake <yasutake.koichi@...panasonic.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org, linux-m32r@...linux-m32r.org,
	linux-m32r-ja@...linux-m32r.org, microblaze-uclinux@...e.uq.edu.au,
	linux-am33-list@...hat.com, linuxppc-dev@...ts.ozlabs.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 10/10] kernel: might_fault does not imply might_sleep

On Sun, May 19, 2013 at 07:40:09PM +0300, Michael S. Tsirkin wrote:
> OK I get it. So let me correct myself. The simple code
> that does something like this under a spinlock:
> >       preempt_disable
> >       pagefault_disable
> >       error = copy_to_user
> >       pagefault_enable
> >       preempt_enable
> >
> is not doing anything wrong and should not get a warning,
> as long as error is handled correctly later.
> Right?

Indeed, but I don't get the point of the preempt_{disable,enable}()
here. Why does it have to disable preemption explicitly here? I thought
all you wanted was to avoid the pagefault handler and make it do the
exception table thing; for that pagefault_disable() is sufficient.
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