Re: 2.6.21-rc7-mm1 BUG at kernel/sched-clock.c:175 init_sched_clock()

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> diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
> index 72c286e..a128717 100644
> --- a/drivers/ata/ahci.c
> +++ b/drivers/ata/ahci.c
> @@ -1369,10 +1369,14 @@ static unsigned int ahci_qc_issue(struct ata_queued_cmd *qc)
>  static void ahci_freeze(struct ata_port *ap)
>  {
> +	void __iomem *mmio = ap->host->iomap[AHCI_PCI_BAR];
>  	void __iomem *port_mmio = ahci_port_base(ap);
>  	/* turn IRQ off */
>  	writel(0, port_mmio + PORT_IRQ_MASK);
> +
> +	/* clear IRQ pending bit */
> +	writel(1 << ap->port_no, mmio + HOST_IRQ_STAT);
>  }

Actually now I think about it the problem is even worse. On older systems
IRQ delivery for SMP machines is asynchronous. So the following can occur

		device raises IRQ line
		IRQ request hits APIC bus

		We write IRQ off
		We clear IRQ pending

		We read back to ensure the write took

		IRQ arrives on CPU APIC

(and yes this happens on dual P3 boxes - NE2K has some fantastic bugs at
one point due to this)
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