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

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Message-ID: <20130522110729.GB5643@redhat.com>
Date:	Wed, 22 May 2013 14:07:29 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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.

Do you think we should drop the preemption point in might_fault?
Only copy_XX_user?
Only __copy_XXX_user ?

-- 
MST
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