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

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Message-ID: <20100331211754.GC32165@parisc-linux.org>
Date:	Wed, 31 Mar 2010 15:17:55 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.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
Subject: Re: start_kernel(): bug: interrupts were enabled early

On Wed, Mar 31, 2010 at 02:05:00PM -0700, H. Peter Anvin wrote:
> What I note is that lib/rwsem-spinlock.c seems to be rather inconsistent
> in its use of spin_lock_irqsave/spin_lock_irqrestore versus
> spin_lock_irq/spin_unlock_irq... in fact, __down_read is the *only*
> place where we use the latter as opposed to the former.
> 
> Is that a bug?  If so, it would certainly explain this behavior.

It's based on down_read() and down_write() not being callable from
interrupt context, or with interrupts disabled (since they can sleep).
up_read(), up_write(), down_read_trylock(), down_write_trylock(),
downgrade_write() can all be called from interrupt context since they
cannot sleep.

-- 
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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