[lkml]   [2011]   [Feb]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    Patch in this message
    SubjectRe: [PATCH] x86: hold mm->page_table_lock while doing vmalloc_sync
    On Thu, Feb 03, 2011 at 12:44:02PM -0800, Jeremy Fitzhardinge wrote:
    > On 02/02/2011 06:48 PM, Andrea Arcangeli wrote:
    > > Hello,
    > >
    > > Larry (CC'ed) found a problem with the patch in subject. When
    > > USE_SPLIT_PTLOCKS is not defined (NR_CPUS == 2) it will deadlock in
    > > ptep_clear_flush_notify in rmap.c because it's sending IPIs with the
    > > page_table_lock already held, and the other CPUs now spins on the
    > > page_table_lock with irq disabled, so the IPI never runs. With
    > > CONFIG_TRANSPARENT_HUGEPAGE=y this deadlocks happens even with
    > > USE_SPLIT_PTLOCKS defined so it become visible but it needs to be
    > > fixed regardless (for NR_CPUS == 2).
    > What's "it" here? Do you mean vmalloc_sync_all? vmalloc_sync_one?
    > What's the callchain?

    Larry just answered to that. If something is unclear let me know. I
    never reproduced it, but it also can happen without THP enabled, you
    just need to set NR_CPUS to 2 during "make menuconfig".

    > > spin_lock_irqsave(pgd_lock) so I guess it's either common code, or
    > > it's superfluous and not another Xen special requirement.
    > There's no special Xen requirement here.

    That was my thought too considering the other archs...

    > mmdrop() can be called from interrupt context, but I don't know if it
    > will ever drop the last reference from interrupt, so maybe you can get
    > away with it.

    Yes the issue is __mmdrop, so it'd be nice to figure if __mmdrop can
    also run from irq (or only mmdrop fast path which would be safe even
    without _irqsave).

    Is this a Xen only thing? Or is mmdrop called from regular
    linux. Considering other archs also _irqsave I assume it's common code
    calling mmdrop (otherwise it means they cut-and-pasted a Xen
    dependency). This comment doesn't really tell me much.

    static void pgd_dtor(pgd_t *pgd)
    unsigned long flags; /* can be called from interrupt context */



    This comment tells the very __mmdrop can be called from irq context,
    not just mmdrop. But I didn't find where yet... Can you tell me?

    > > @@ -247,7 +248,7 @@ void vmalloc_sync_all(void)
    > > if (!ret)
    > > break;
    > > }
    > > - spin_unlock_irqrestore(&pgd_lock, flags);
    > > + spin_unlock(&pgd_lock, flags);
    > Urp. Did this compile?

    Yes it builds and it also runs fine still (I left it running since I
    posted the email and no problems yet, but this may not be reproducible
    and we really need to know who calls __mmdrop from irq context to
    tell). The above is under CONFIG_X86_32 and I did a 64bit build ;).

    I'm not reposting a version that builds for 32bit x86 too until we
    figure out the mmdrop thing...


     \ /
      Last update: 2011-02-04 02:23    [W:0.022 / U:13.028 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site