On Mon, Nov 05, 2018 at 10:49:21PM +0000, Will Deacon wrote:
> diff --git a/include/asm-generic/bitops/lock.h
> b/include/asm-generic/bitops/lock.h
> index 3ae021368f48..9de8d3544630 100644
> --- a/include/asm-generic/bitops/lock.h
> +++ b/include/asm-generic/bitops/lock.h
> @@ -6,6 +6,15 @@
> #include <linux/compiler.h>
> #include <asm/barrier.h>
>
> +static inline void spin_until_bit_unlock(unsigned int nr,
> + volatile unsigned long *p)
> +{
> + unsigned long mask = BIT_MASK(bitnum);
> +
> + p += BIT_WORD(nr);
> + smp_cond_load_relaxed(p, VAL & mask);
> +}
> +
> /**
> * test_and_set_bit_lock - Set a bit and return its old value, for lock
> * @nr: Bit to set
> diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
> index bbc4730a6505..d711c62e718c 100644
> --- a/include/linux/bit_spinlock.h
> +++ b/include/linux/bit_spinlock.h
> @@ -26,9 +26,7 @@ static inline void bit_spin_lock(int bitnum, unsigned long
> *addr)
> #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
> while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
> preempt_enable();
> - do {
> - cpu_relax();
> - } while (test_bit(bitnum, addr));
> + spin_until_bit_unlock(bitnum, addr);
> preempt_disable();
> }
> #endif
Yes, that's much better. Ideally though, we'd get rid of bit spinlocks
that have significant enough contention for this to matter.
