linux-kernel - Re: [RFC] [PATCH] Pre-emption control for userspace

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Message-ID: <alpine.DEB.2.02.1403061530470.18573@ionos.tec.linutronix.de>
Date:	Thu, 6 Mar 2014 15:33:04 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Khalid Aziz <khalid.aziz@...cle.com>,
	Ingo Molnar <mingo@...nel.org>, akpm@...ux-foundation.org,
	andi.kleen@...el.com, rob@...dley.net, viro@...iv.linux.org.uk,
	oleg@...hat.com, venki@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace



On Thu, 6 Mar 2014, Peter Zijlstra wrote:

> On Thu, Mar 06, 2014 at 02:45:00PM +0100, Rasmus Villemoes wrote:
> > Peter Zijlstra <peterz@...radead.org> writes:
> > 
> > > On Thu, Mar 06, 2014 at 02:24:43PM +0100, Rasmus Villemoes wrote:
> > >> Is it possible to implement non-sleeping versions of {get,put}_user()?
> > >
> > > __{get,put}_user()
> > 
> > Huh?
> > 
> > arch/x86/include/asm/uaccess.h:
> > 
> > /**
> >  * __get_user: - Get a simple variable from user space, with less checking.
> >  * @x:   Variable to store result.
> >  * @ptr: Source address, in user space.
> >  *
> >  * Context: User context only.  This function may sleep.
> > 
> > What am I missing?
> 
> __get_user() -> __get_user_nocheck() -> __get_user_size() -> __get_user_asm()
> 
> And __get_user_asm() seems to generate the required .fixup section for
> this to work in pagefault_disable() context.

Well, it still might sleep if you're using it in page fault enabled
context. So the documentation of that function sucks.

Thanks,

	tglx

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