linux-kernel - Re: start_kernel(): bug: interrupts were enabled early

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Date:	Thu, 1 Apr 2010 05:23:52 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	David Howells <dhowells@...hat.com>,
	Russell King <rmk@....linux.org.uk>,
	Yinghai Lu <yinghai@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rabin Vincent <rabin@....in>,
	lkml <linux-kernel@...r.kernel.org>, hpa@...or.com,
	penberg@...helsinki.fi, cl@...ux-foundation.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-arch@...r.kernel.org
Subject: Re: start_kernel(): bug: interrupts were enabled early

On Thu, Apr 01, 2010 at 10:41:11AM +0100, Jamie Lokier wrote:
> David Howells wrote:
> > Russell King <rmk@....linux.org.uk> wrote:
> > > We use the standard generic kernel implementation.  Is x86 different? ;)
> > 
> > The optimised fast paths used on x86 rwsems don't disable interrupts.
> 
> Any reason not to use the same technique for all the archs - plus the
> trick used in arch/armkernel/entry-armv.S:__kuser_cmpxchg for those
> archs which don't have atomic instructions or ll/sc?

Assuming you're talking about the __LINUX_ARM_ARCH__ < 6 + CONFIG_MMU
case, then this only works for uniprocessor machines.

> If the problem here is _only_ semaphores, and the above might make
> semaphores faster anyway, perhaps it's a solution.

You trade off a bit of overhead in the semaphore path for a bit of
overhead in the interrupt path.  We probably take more sempahores than
we do interrupts, so it's probably worthwhile.  Still, cmpxchg() needs
to be SMP-safe.

Realistically, this isn't something that can be done in generic code.
It has to be done in arch-specific code.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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