[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] Re: [PATCH] fix pgd_lock deadlock
>>> 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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