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xen-devel
[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.
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