2022-07-14 18:48:55

by Uros Bizjak

Subject: [PATCH] random: Use try_cmpxchg in _credit_init_bits

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
_credit_init_bits. x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: "Jason A. Donenfeld" <[email protected]>
---
drivers/char/random.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index e3dd1dd3dd22..78e690f0f9f6 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -643,10 +643,10 @@ static void __cold _credit_init_bits(size_t bits)

add = min_t(size_t, bits, POOL_BITS);

+ orig = READ_ONCE(input_pool.init_bits);
do {
- orig = READ_ONCE(input_pool.init_bits);
new = min_t(unsigned int, POOL_BITS, orig + add);
- } while (cmpxchg(&input_pool.init_bits, orig, new) != orig);
+ } while (!try_cmpxchg(&input_pool.init_bits, &orig, new));

if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
--
2.35.3

2022-07-16 19:08:15

by Jason A. Donenfeld

Subject: Re: [PATCH] random: Use try_cmpxchg in _credit_init_bits

Hi Uros,

On Thu, Jul 14, 2022 at 08:28:22PM +0200, Uros Bizjak wrote:
> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
> _credit_init_bits. x86 CMPXCHG instruction returns success in ZF flag,
> so this change saves a compare after cmpxchg (and related move
> instruction in front of cmpxchg).
>
> Also, try_cmpxchg implicitly assigns old *ptr value to "old"
> when cmpxchg fails, enabling further code simplifications.
>
> No functional change intended.

Applied as:
https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git/commit/?id=6a66e096619e3a737297eb2f9f535a287af43a95

Thanks for the patch.

Jason