Re: [GIT PULL] parisc huge page support for v4.4

[John David Anglin]

On 2016-01-04 4:24 PM, Mikulas Patocka wrote:
> On Sat, 26 Dec 2015, Helge Deller wrote:
>
> > On 26.12.2015 13:09, Mikulas Patocka wrote:
> >
> > > BTW. I looked at this in arch/parisc/mm/hugetlbpage.c:set_huge_pte_at
> > > "*ptep = entry;" and it seems like a bad bug. PA-RISC doesn't have atomic
> > > instructions to modify page table entries, so it takes spinlock in the TLB
> > > handler and modifies the page table entry non-atomically. If you modify
> > > the page table entry without the spinlock, you may race with TLB handler
> > > on another CPU and your modification may be lost.
> > Right.
> >
> > > The comment says something about double locking on pa_tlb_lock, but
> > > pa_tlb_lock isn't held when that function is called.
> > I have a work-in-progress patch for that in one of my trees, e.g.:
> > http://git.kernel.org/cgit/linux/kernel/git/deller/parisc-linux.git/commit/?h=parisc-next&id=5c76b525cbdb097401f46522b27b1eb6244f34f9
> > It's lightly tested though.
> >
> > Helge
> I tested the patch and it works OK for me so far.
>
> BTW. what happens if some kernel code takes the TLB spinlock and then TLB
> miss in kernel space happens? (it would attempt to lock the spinlock
> recursively) Is it assumed that the TLB is big enough that this can't
> happen?
No.  If you look at the TLB handler, you will see that locking is not 
done for misses in
kernel space.  So, this deadlock doesn't occur.

Dave

--
John David Anglin  dave.anglin@xxxxxxxx

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