linux-kernel - Re: [PATCH v2 00/10] uaccess: better might_sleep/might

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Date:	Wed, 22 May 2013 13:27:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Arnd Bergmann <arnd@...db.de>, 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,
	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 00/10] uaccess: better might_sleep/might_fault behavior

On Wed, May 22, 2013 at 02:07:29PM +0300, Michael S. Tsirkin wrote:
> On Wed, May 22, 2013 at 12:19:16PM +0200, Peter Zijlstra wrote:
> > On Wed, May 22, 2013 at 11:25:36AM +0200, Arnd Bergmann wrote:
> > > Calling might_fault() for every __get_user/__put_user is rather expensive
> > > because it turns what should be a single instruction (plus fixup) into an
> > > external function call.
> > 
> > We could hide it all behind CONFIG_DEBUG_ATOMIC_SLEEP just like
> > might_sleep() is. I'm not sure there's a point to might_fault() when
> > might_sleep() is a NOP.
> 
> The patch that you posted gets pretty close.
> E.g. I'm testing this now:
> +#define might_fault() do { \
> +       if (_might_fault()) \
> +               __might_sleep(__FILE__, __LINE__, 0); \
> +       might_resched(); \
> +} while(0)
> 
> So if might_sleep is a NOP, __might_sleep and might_resched are NOPs
> so compiler will optimize this all out.
> 
> However, in a related thread, you pointed out that might_sleep is not a NOP if
> CONFIG_PREEMPT_VOLUNTARY is set, even without CONFIG_DEBUG_ATOMIC_SLEEP.

Oh crud yeah.. and you actually need that _might_fault() stuff for that
too. Bugger.

Yeah, I wouldn't know what the effects of dropping ita (for the copy
functions) would be, VOLUNTARY isn't a preemption mode I ever use (even
though it seems most distros default to it).


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