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>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a1af90b..0c6568a 100644 (file)
--- 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. */