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

[Alan Cox]

>  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);


Neither of those two are going to help in all cases. They are PCI writes
to mmio so they are posted. You don't know they hit the device until a
read cycle coming back has completed, so you need to read something after
doing this to be sure.
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