lkml.org 
[lkml]   [2008]   [Mar]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: preempt bug in set_pmd_pfn?
Hugh Dickins wrote:
> On Wed, 5 Mar 2008, Jeremy Fitzhardinge wrote:
>
>> If a thread goes from processor A -> B -> A, where A is first preempted
>> between a pagetable update and a tlb flush, then the second time the thread
>> runs on A may run with a stale tlb (if in the meantime A has either been idle
>> or only running kernel threads).
>>
>
> Right, thanks, because __flush_tlb_one opts out of the full active_mm
> checking which goes on for userspace mms (which covers, for example,
> the case of preemption before dup_mmap's flush_tlb_mm).
>
> But is there actually a case where there's a problem? So far as I can see,
> set_pmd_pfn is there solely for discontig_32's __init remap_numa_kva; and
> set_pte_pfn is there solely for set_fixmap, which operates on a per-cpu
> area of pagetable, which would already be in bigger trouble if preemption
> to another cpu were possible.

Yes, I meant set_pte_pfn; set_pmd_pfn is a typo.

Fixmap slots are global, not percpu; you may be thinking of kmap_atomic,
which reserves percpu fixmap slots for its use. Most uses of set_fixmap
are early in boot, where preemption (or other CPUs) isn't a factor. The
exception is mapping the compat vdso global mapping. However, that is
special-cased anyway, since the set_fixmap is followed by an explicit
all-cpu tlb flush.

It seems to me that the correct fix is to just make __set_fixmap disable
preemption for its duration; it probably doesn't make much difference
for the native case, and it makes Xen happy.

J


\
 
 \ /
  Last update: 2008-03-05 20:27    [W:0.274 / U:0.080 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site