Re: 2.6.24-git: kmap_atomic() WARN_ON()

Thomas Gleixner
2008-02-25T22:03:28Z

On Mon, 25 Feb 2008, Andrew Morton wrote:
> On Mon, 25 Feb 2008 15:08:35 -0500 Jeff Garzik <jgarzik@pobox.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