On Tue, Dec 05 2017, Matthew Wilcox wrote:
> On Tue, Dec 05, 2017 at 07:11:00AM +1100, NeilBrown wrote:
>> As we don't seem to be pursuing this possibility is probably isn't very
>> important, but I'd like to point out that the original fix isn't a true
>> fix.
>> It just sorts a shared group_info early. This does not stop corruption.
>> Every time a thread calls set_groups() on that group_info it will be
>> sorted again.
>> The sort algorithm used is the heap sort, and a heap sort always moves
>> elements in the array around - it does not leave a sorted array
>> untouched (unlike e.g. the quick sort which doesn't move anything in a
>> sorted array).
>> So it is still possible for two calls to groups_sort() to race.
>> We *need* to move groups_sort() out of set_groups().
>
> It must be relatively common to sort an already-sorted array. I wonder
> if something like this patch would be worthwhile?
>
> I have deliberately broken this patch so it can't be applied. I haven't
> tested it, and for all I know, I got the sign of cmp_func wrong.
>
> diff --git a/lib/sort.c b/lib/sort.c
> index d6b7a202b0b6..2b527fde6dad 100644
> --- a/lib/sort.c
> +++ b/lib/sort.c
> @@ -75,7 +75,14 @@ void sort(void *base, size_t num, size_t size,
> swap_func = generic_swap;
> }
>
> - /* heapify */
> + /* Do not sort an already-sorted array */
> + for (c = 0; c < (n - size); c += size) {
> + if (cmp_func(base + c, base + c + size) < 0)
> + goto heapify;
> + }
> + return;
> +
> +heapify:
> for ( ; i >= 0; i -= size) {
> for (r = i; r * 2 + size < n; r = c) {
> c = r * 2 + size;
I wondered about this possibility...
It is a clear win from a defensive-programming perspective.
Adding a small overhead to every sort call isn't ideal, but I doubt it
is noticeable (typically 2 or 3 tests I suspect).
I probably wouldn't advocate this approach, but I certainly wouldn't
fight it.
I *do* like the way you changed a comment to a label!
Thanks,
NeilBrown
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