Re: [PATCH v2] mm/shuffle.c: Fix races in add_to_free_area_random()

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Mar 17, 2020 at 6:44 PM George Spelvin <lkml@sdf.org> wrote:
>
> The old code had separate "rand" and "rand_count" variables,
> which could get out of sync with bad results.
>
> In the worst case, two threads would see rand_count == 1 and
> both decrement it, resultint in rand_count = 255 and rand being
> filled with zeros for the next 255 calls.
>
> Instead, pack them both into a single, atomically updatable,
> variable.  This makes it a lot easier to reason about race
> conditions.  They are still there - the code deliberately eschews
> locking - but basically harmless on the rare occasions that
> they happen.
>
> Second, use READ_ONCE and WRITE_ONCE.  Without them, we are deep
> in the land of nasal demons.  The compiler would be free to spill
> temporaries to the static variables in arbitrary perverse ways
> and create hard-to-find bugs.
>
> (Alternatively, we could declare the static variable "volatile",
> one of the few places in the Linux kernel that would be correct,
> but it would probably annoy Linus.)
>
> Third, use long rather than u64.  This not only keeps the
> state atomically updatable, it also speeds up the fast path
> on 32-bit machines.  Saving at least three instructions on
> the fast path (one load, one add-with-carry, and one store)
> is worth exchanging one call to get_random_u64 for two
> calls to get_random_u32.  The fast path of get_random_* is
> less than the 3*64 = 192 instructions saved, and the slow
> path happens every 64 bytes so isn't affectrd by the change.
>
> I've tried a few variants.  Keeping random lsbits with
> a most-significant end marker, and using an explicit bool
> flag rather than testing r both increase code size slightly.
>
>                 x86_64  i386
> This code                94      95
> Explicit bool           103      99
> Lsbits           99     101
> Both             96     100
>
> Signed-off-by: George Spelvin <lkml@sdf.org>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org
> ---
>  mm/shuffle.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/mm/shuffle.c b/mm/shuffle.c
> index e0ed247f8d90..4ba3ba84764d 100644
> --- a/mm/shuffle.c
> +++ b/mm/shuffle.c
> @@ -186,22 +186,28 @@ void __meminit __shuffle_free_memory(pg_data_t *pgdat)
>  void add_to_free_area_random(struct page *page, struct free_area *area,
>                 int migratetype)
>  {
> -       static u64 rand;
> -       static u8 rand_bits;
> +       static long rand;       /* 0..BITS_PER_LONG-1 buffered random bits */
> +       unsigned long r = READ_ONCE(rand), rshift = r << 1;;
>
>         /*
> -        * The lack of locking is deliberate. If 2 threads race to
> -        * update the rand state it just adds to the entropy.
> +        * rand holds some random msbits, with a 1 bit appended, followed
> +        * by zero-padding in the lsbits.  This allows us to maintain
> +        * the pre-generated bits and the count of bits in a single,
> +        * atomically updatable, variable.
> +        *
> +        * The lack of locking is deliberate. If two threads race to
> +        * update the rand state it just adds to the entropy.  The
> +        * worst that can happen is a random bit is used twice, or
> +        * get_random_long is called redundantly.
>          */
> -       if (rand_bits == 0) {
> -               rand_bits = 64;
> -               rand = get_random_u64();
> +       if (unlikely(rshift == 0)) {
> +               r = get_random_long();
> +               rshift = r << 1 | 1;

You might want to wrap the "r << 1" in parenthesis. Also you could
probably use a + 1 instead of an | 1.

>         }
> +       WRITE_ONCE(rand, rshift);
>
> -       if (rand & 1)
> +       if ((long)r < 0)

One trick you might be able to get away with here is to actually
compare r to rshift. "If (rshift <= r)" should give you the same
result. This works since what you are essentially doing is just adding
r to itself so if you overflow rshift will be equal to at most r - 1.
However with the addition of the single bit in the rshift == 0 case it
could potentially be equal in the unlikely case of r being all 1's.

>                 add_to_free_area(page, area, migratetype);
>         else
>                 add_to_free_area_tail(page, area, migratetype);
> -       rand_bits--;
> -       rand >>= 1;
>  }
> --
> 2.26.0.rc2
>