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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] Re: Question about x86/mm/gup.c's use of disabled interrupts
Jeremy Fitzhardinge wrote: I thought you were concerned about cpu 0 doing a gup_fast(), cpu 1 doing P->N, and cpu 2 doing N->P. In this case cpu 2 is waiting on the pte lock.The issue is that if cpu 0 is doing a gup_fast() and other cpus are doing P->P updates, then gup_fast() can potentially get a mix of old and new pte values - where P->P is any aggregate set of unsynchronized P->N and N->P operations on any number of other cpus. Ah, but if every P->N is followed by a tlb flush, then disabling interrupts will hold off any following N->P, allowing gup_fast to get a consistent pte snapshot. Right. Hm, awkward if flush_tlb_others doesn't IPI... How can it avoid flushing the tlb on cpu [01]? It's it's gup_fast()ing a pte, it may as well load it into the tlb. Simplest fix is to make gup_get_pte() a pvop, but that does seem like putting a red flag in front of an inner-loop hotspot, or something...The per-cpu tlb-flush exclusion flag might really be the way to go. I don't see how it will work, without changing Xen to look at the flag?local_irq_disable() is used here to lock out a remote cpu, I don't see why deferring the flush helps. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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