Commit b7a68f67 authored Jul 14, 2022 by Uros Bizjak Committed by Jason A. Donenfeld Jul 18, 2022

random: use try_cmpxchg in _credit_init_bits



Use `!try_cmpxchg(ptr, &orig, new)` instead of `cmpxchg(ptr, orig, new)
!= orig` in _credit_init_bits. This has two benefits:

- The x86 cmpxchg instruction returns success in the ZF flag, so this
  change saves a compare after cmpxchg, as well as a related move
  instruction in front of cmpxchg.

- try_cmpxchg implicitly assigns the *ptr value to &orig when cmpxchg
  fails, enabling further code simplifications.

This patch has no functional change.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

parent b8ac29b4

drivers/char/random.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -643,10 +643,10 @@ static void __cold _credit_init_bits(size_t bits)

		add = min_t(size_t, bits, POOL_BITS);

		do {
		orig = READ_ONCE(input_pool.init_bits);
		do {
		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. */