On Tue, Mar 17, 2020 at 6:44 PM George Spelvin <lkml@xxxxxxx> 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@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
> 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)) {
I had the impression that unless unlikely is "mostly never" then it
can do more harm than good. Is a branch guaranteed to be taken every
BITS_PER_LONG'th occurrence really a candidate for unlikely()
annotation?
Otherwise this change looks good to me.
> + r = get_random_long();
> + rshift = r << 1 | 1;
> }
> + WRITE_ONCE(rand, rshift);
>
> - if (rand & 1)
> + if ((long)r < 0)
> add_to_free_area(page, area, migratetype);
> else
> add_to_free_area_tail(page, area, migratetype);
> - rand_bits--;
> - rand >>= 1;
> }
> --
> 2.26.0.rc2
