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

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Message-Id: <20100401102744.a4e6f24d.akpm@linux-foundation.org>
Date:	Thu, 1 Apr 2010 10:27:44 -0400
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Yinghai Lu <yinghai@...nel.org>,
	Rabin Vincent <rabin@....in>,
	lkml <linux-kernel@...r.kernel.org>, penberg@...helsinki.fi,
	cl@...ux-foundation.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-arch@...r.kernel.org, David Howells <dhowells@...hat.com>
Subject: Re: start_kernel(): bug: interrupts were enabled early

On Thu, 1 Apr 2010 09:13:31 -0700 (PDT) Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Wed, 31 Mar 2010, H. Peter Anvin wrote:
> > 
> > The obvious way to fix this would be to use
> > spin_lock_irqsave..spin_lock_irqrestore in __down_read as well as in the
> > other locations; I don't have a good feel for what the cost of doing so
> > would be, though.  On x86 it's fairly expensive simply because the only
> > way to save the state is to push it on the stack, which the compiler
> > doesn't deal well with, but this code isn't used on x86.
> 
> I think that's what we should just do, with a good comment both in the 
> code and the changelog. I'm not entirely happy with it, because obviously 
> it's conceptually kind of dubious to take a lock with interrupts disabled 
> in the first place, but this is not a new issue per se.
> 
> The whole bootup code is special, and we already make similar guarantees 
> about memory allocators and friends - just because it's too dang painful 
> to have some special code that does GFP_ATOMIC for early bootup when the 
> same code is often shared and used at run-time too.
> 
> So we've accepted that people can do GFP_KERNEL allocations and we won't 
> care about them if we're in the boot phase (and suspend/resume), and we 
> have that whole 'gfp_allowed_mask' thing for that.
> 
> I think this probably falls under exactly the same heading of "not pretty, 
> but let's not blow up".
> 
> So making the slow-path do the spin_[un]lock_irq{save,restore}() versions 
> sounds like the right thing. It won't be a performance issue: it _is_ the 
> slow-path, and we're already doing the expensive part (the spinlock itself 
> and the irq thing).

It's actually on the fastpath for lib/rwsem-spinlock.c.

> So ACK on the idea. Who wants to write the trivial patch and test it? 
> Preferably somebody who sees the problem in the first place - x86 should 
> not be impacted, since the irq-disabling slow-path should never be hit 
> without contention anyway (and contention cannot/mustnot happen for this 
> case).
> 
> 			Linus
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