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

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Message-ID: <20130519203551.GA27708@redhat.com>
Date:	Sun, 19 May 2013 23:35:51 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.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 04:23:22PM -0400, Steven Rostedt wrote:
> On Sun, 2013-05-19 at 19:40 +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?
> 
> I came in mid thread and I don't know the context.

The context is that I want to change might_fault
from might_sleep to
might_sleep_if(!in_atomic())
so that above does not trigger warnings even with
CONFIG_DEBUG_ATOMIC_SLEEP enabled.


> Anyway, the above
> looks to me as you just don't want to sleep.

Exactly. upstream we can just do pagefault_disable but to make
this code -rt ready it's best to do preempt_disable as well.

> If you try to copy data to
> user space that happens not to be currently mapped for any reason, you
> will get an error. Even if the address space is completely valid. Is
> that what you want?
> 
> -- Steve
> 

Yes, this is by design.
We detect that and bounce the work to a thread outside
any locks.

Thanks,

-- 
MST
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