linux-kernel - Re: 2.6.24-git: kmap_atomic() WARN

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Message-ID: <alpine.LFD.1.00.0802252242290.7583@apollo.tec.linutronix.de>
Date:	Mon, 25 Feb 2008 23:01:59 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Jeff Garzik <jgarzik@...ox.com>,
	Björn Steinbrink <B.Steinbrink@....de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.24-git: kmap_atomic() WARN_ON()

On Mon, 25 Feb 2008, Andrew Morton wrote:
> On Mon, 25 Feb 2008 15:08:35 -0500 Jeff Garzik <jgarzik@...ox.com> wrote:
> > There are plenty of drivers that do the same thing that ahci does, in 
> > terms of interrupt handler locking...  and I will definitely push back 
> > on efforts to convert otherwise-100%-safe spin_lock() into 
> > spin_lock_irqsave() just to quiet lockdep.
> > 
> > Very interesting email, thanks...
> > 
> 
> I suspect this is a bug in my old kmap_atomic debugging patch.  It doesn't
> know about the implicit irq-disablememnt which interrupt handlers enjoy.  I
> don't think...

I suspect here is confusion. The implicit irq-disablement of lockdep
is actually hiding the warning.

The code which emits the warning is:

        if (type == KM_IRQ0 || type == KM_IRQ1 || type == KM_BOUNCE_READ ||
                        type == KM_BIO_SRC_IRQ || type == KM_BIO_DST_IRQ) {
                if (!irqs_disabled()) {
                        WARN_ON(1);
                        warn_count--;
                }

It checks for _NOT_ irqs_disabled. The calling code is
ata_scsi_rbuf_get() which calls with:

     buf = kmap_atomic(sg_page(sg), KM_IRQ0) + sg->offset;

This happens with interrupts enabled. So the warning is according to
the well documented km_type enum and the equally well documented
highmem debug code correct.

Bjoern decoded it very well, just Jeff jumped to very interesting
conclusions.

Thanks,

	tglx
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