Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed

[Will Deacon <will.deacon@arm.com>]

[+PeterZ -- please include him on stuff like this]

Hi Gao,

On Tue, Oct 30, 2018 at 02:04:41PM +0800, Gao Xiang wrote:
> It is better to use wrapped smp_cond_load_relaxed
> instead of open-coded busy waiting for bit_spinlock.
> 
> Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
> ---
> 
> change log v2:
>  - fix the incorrect expression !(VAL >> (bitnum & (BITS_PER_LONG-1)))
>  - the test result is described in the following reply.

Please include the results in the commit message, so that this change is
justified.

> diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
> index bbc4730a6505..d5f922b5ffd9 100644
> --- a/include/linux/bit_spinlock.h
> +++ b/include/linux/bit_spinlock.h
> @@ -15,22 +15,19 @@
>   */
>  static inline void bit_spin_lock(int bitnum, unsigned long *addr)
>  {
> -	/*
> -	 * Assuming the lock is uncontended, this never enters
> -	 * the body of the outer loop. If it is contended, then
> -	 * within the inner loop a non-atomic test is used to
> -	 * busywait with less bus contention for a good time to
> -	 * attempt to acquire the lock bit.
> -	 */
> -	preempt_disable();
>  #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));
> +	const unsigned int bitshift = bitnum & (BITS_PER_LONG - 1);
> +
> +	while (1) {
> +		smp_cond_load_relaxed(&addr[BIT_WORD(bitnum)],
> +				      !((VAL >> bitshift) & 1));
>  		preempt_disable();
> +		if (!test_and_set_bit_lock(bitnum, addr))
> +			break;
> +		preempt_enable();
>  	}
> +#else
> +	preempt_disable();

This appears to introduce a bunch of overhead for the uncontended fastpath.
How about the much-simpler-but-completely-untested (tm) patch below?

Will

--->8

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