preempted dup_mm misses TLB invalidate
nfortino at nvidia.com
Wed Jul 17 17:03:34 EDT 2013
On 7/17/2013 1:34 PM, Russell King - ARM Linux wrote:
> Even if you stopped all threads on entry to a fork, the same problem
> exists - at the point that you stopped the other threads, how do you know
> what data they've written to memory?
> What I'm pointing out here is that in this situation, the data visible to
> the child process is unpredictable.
I agree the data visible to the child process is inherently
unpredictable. If you stop all process threads on a fork, however, you
do preserve the invariant that memory accesses are not seen out of
order. In your memcopy case, it is indeterminate how much of the memcopy
has completed, but it is known that later pages in the memcopy will only
have been written if prior pages have been updated.
With the current kernel configuration, you can have holes. Any page
which hits in the TLB has the memcopy data appear in the forked process.
Any page which misses in the TLB will not appear in the forked process.
The choice of which pages get memcopy data in the forked process will
appear random based on TLB contents, a behavior you cannot have if you
freeze threads on a fork.
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