[Xen-devel] Re: [PATCH] fix pgd_lock deadlock

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 24.02.11 at 05:22, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
> On Tue, Feb 22, 2011 at 02:22:53PM +0000, Jan Beulich wrote:
>> >>> On 22.02.11 at 14:49, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
>> > On Tue, Feb 22, 2011 at 07:48:54AM +0000, Jan Beulich wrote:
>> >> A possible alternative would be to acquire the page table lock
>> >> in vmalloc_sync_all() only in the Xen case (perhaps by storing
>> >> NULL into page->index in pgd_set_mm() when not running on
>> >> Xen). This is utilizing the fact that there aren't (supposed to
>> >> be - for non-pvops this is definitely the case) any TLB flush IPIs
>> >> under Xen, and hence the race you're trying to fix doesn't
>> >> exist there (while non-Xen doesn't need the extra locking).
>> > 
>> > That's sure ok with me. Can we use a global runtime to check if the
>> > guest is running under Xen paravirt, instead of passing that info
>> > through page->something?
>> 
>> If everyone's okay with putting a couple of "if (xen_pv_domain())"
>> into mm/fault.c - sure. I would have thought that this wouldn't be
>> liked, hence the suggestion to make this depend on seeing the
>> backlink be non-NULL.
> 
> What about this? The page->private logic gets optimized away at
> compile time with XEN=n.
> 
> The removal of _irqsave from pgd_lock, I'll delay it as it's no bugfix
> anymore.
> 
> ===
> Subject: xen: stop taking the page_table_lock with irq disabled
> 
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> 
> It's forbidden to take the page_table_lock with the irq disabled or if there's
> contention the IPIs (for tlb flushes) sent with the page_table_lock held will
> never run leading to a deadlock.
> 
> Only Xen needs the page_table_lock and Xen won't need IPI TLB flushes hence 
> the deadlock doesn't exist for Xen.

Looks reasonable to me, except for the implementation no longer
matching subject and description (the lock still gets taken with
IRQs disabled, just that - as far as we can tell so far - doesn't
matter for Xen).

With the conditional on the reader side I also wonder whether
the conditional on the writer side is really a good thing to have,
considering that generally distro kernels are likely to have XEN
enabled.

Jan


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