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