bit_spinlock: don't play preemption games inside the busy loop

When we are waiting for the bit-lock to be released, and are looping
over the 'cpu_relax()' should not be doing anything else - otherwise we
miss the point of trying to do the whole 'cpu_relax()'.

Do the preemption enable/disable around the loop, rather than inside of
it.

Noticed when I was looking at the code generation for the dcache
__d_drop usage, and the code just looked very odd.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
index e612575..b4326bf 100644 (file)
--- a/include/linux/bit_spinlock.h
+++ b/include/linux/bit_spinlock.h
@@ -23,11 +23,11 @@ static inline void bit_spin_lock(int bitnum, unsigned long *addr)
        preempt_disable();
 #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
        while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
-               while (test_bit(bitnum, addr)) {
-                       preempt_enable();
+               preempt_enable();
+               do {
                        cpu_relax();
-                       preempt_disable();
-               }
+               } while (test_bit(bitnum, addr));
+               preempt_disable();
        }
 #endif
        __acquire(bitlock);