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    SubjectRe: [PATCH] fix pgd_lock deadlock
    On Tue, 15 Feb 2011, Thomas Gleixner wrote:

    > On Tue, 15 Feb 2011, Andrea Arcangeli wrote:
    > > On Tue, Feb 15, 2011 at 08:26:51PM +0100, Thomas Gleixner wrote:
    > >
    > > With NR_CPUs < 4, or with THP enabled, rmap.c will do
    > > spin_lock(&mm->page_table_lock) (or pte_offset_map_lock where the lock
    > > is still mm->page_table_lock and not the PT lock). Then it will send
    > > IPIs to flush the tlb of the other CPUs.
    > >
    > > But the other CPU is running the vmalloc_sync_all, and it is trying to
    > > take the page_table_lock with irq disabled. It will never take the
    > > lock because the CPU waiting the IPI delivery holds it. And it will
    > > never run the IPI because it has irqs disabled.
    >
    > Ok, that makes sense :)
    >
    > > Now the big question is if anything is taking the pgd_lock from
    > > irqs. Normal testing could never reveal it as even if it happens it
    > > has a slim chance to happen while the pgd_lock is already hold by
    > > normal kernel context. But the VM_BUG_ON(in_interrupt()) should
    > > hopefully have revealed it already if it ever happened, I hope.
    > >
    > > Clearly we could try to fix it in other ways, but still if there's no
    > > reason to do the _irqsave this sounds a good idea to apply my fix
    > > anyway.
    >
    > Did you try with DEBUG_PAGEALLOC, which is calling into cpa quite a
    > lot?

    Another thing. You check for in_interrupt(), but what makes sure that
    the code which takes pgd_lock is never taken with interrupts disabled
    except during early boot ?

    Thanks,

    tglx


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