linux-kernel - Re: [PATCH 2/3, V2] kernel: Move groups_sort to the caller of set

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Date:   Wed, 06 Dec 2017 09:05:15 +1100
From:   NeilBrown <neilb@...e.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Thiago Rafael Becker <thiago.becker@...il.com>,
        bfields@...ldses.org, linux-nfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3, V2] kernel: Move groups_sort to the caller of set_groups.

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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