Re: start_kernel(): bug: interrupts were enabled early

From: Andrew Morton
Date: Thu Apr 01 2010 - 13:30:31 EST

On Thu, 1 Apr 2010 09:13:31 -0700 (PDT) Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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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