Subject: Re: [PATCH v6 1/9] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

From: Boqun Feng <[email-masked]>
Date: Wed, 7 Apr 2021 00:51:56 +0800

Hi,

On Wed, Mar 31, 2021 at 02:30:32PM +0000, [email-masked] wrote:
> From: Guo Ren <[email-masked]>
> 
> Some architectures don't have sub-word swap atomic instruction,
> they only have the full word's one.
> 
> The sub-word swap only improve the performance when:
> NR_CPUS < 16K
>  *  0- 7: locked byte
>  *     8: pending
>  *  9-15: not used
>  * 16-17: tail index
>  * 18-31: tail cpu (+1)
> 
> The 9-15 bits are wasted to use xchg16 in xchg_tail.
> 
> Please let architecture select xchg16/xchg32 to implement
> xchg_tail.
> 

If the architecture doesn't have sub-word swap atomic, won't it generate
the same/similar code no matter which version xchg_tail() is used? That
is even CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32=y, xchg_tail() acts
similar to an xchg16() implemented by cmpxchg(), which means we still
don't have forward progress guarantee. So this configuration doesn't
solve the problem.

I think it's OK to introduce this config and don't provide xchg16() for
risc-v. But I don't see the point of converting other architectures to
use it.

Regards,
Boqun

> Signed-off-by: Guo Ren <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Ingo Molnar <[email-masked]>
> Cc: Waiman Long <[email-masked]>
> Cc: Arnd Bergmann <[email-masked]>
> Cc: Anup Patel <[email-masked]>
> ---
>  kernel/Kconfig.locks       |  3 +++
>  kernel/locking/qspinlock.c | 46 +++++++++++++++++++++-----------------
>  2 files changed, 28 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
> index 3de8fd11873b..d02f1261f73f 100644
> --- a/kernel/Kconfig.locks
> +++ b/kernel/Kconfig.locks
> @@ -239,6 +239,9 @@ config LOCK_SPIN_ON_OWNER
>  config ARCH_USE_QUEUED_SPINLOCKS
>  	bool
>  
> +config ARCH_USE_QUEUED_SPINLOCKS_XCHG32
> +	bool
> +
>  config QUEUED_SPINLOCKS
>  	def_bool y if ARCH_USE_QUEUED_SPINLOCKS
>  	depends on SMP
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index cbff6ba53d56..4bfaa969bd15 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -163,26 +163,6 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
>  	WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL);
>  }
>  
> -/*
> - * xchg_tail - Put in the new queue tail code word & retrieve previous one
> - * @lock : Pointer to queued spinlock structure
> - * @tail : The new queue tail code word
> - * Return: The previous queue tail code word
> - *
> - * xchg(lock, tail), which heads an address dependency
> - *
> - * p,*,* -> n,*,* ; prev = xchg(lock, node)
> - */
> -static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> -{
> -	/*
> -	 * We can use relaxed semantics since the caller ensures that the
> -	 * MCS node is properly initialized before updating the tail.
> -	 */
> -	return (u32)xchg_relaxed(&lock->tail,
> -				 tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
> -}
> -
>  #else /* _Q_PENDING_BITS == 8 */
>  
>  /**
> @@ -206,6 +186,30 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
>  {
>  	atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
>  }
> +#endif /* _Q_PENDING_BITS == 8 */
> +
> +#if _Q_PENDING_BITS == 8 && !defined(CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32)
> +/*
> + * xchg_tail - Put in the new queue tail code word & retrieve previous one
> + * @lock : Pointer to queued spinlock structure
> + * @tail : The new queue tail code word
> + * Return: The previous queue tail code word
> + *
> + * xchg(lock, tail), which heads an address dependency
> + *
> + * p,*,* -> n,*,* ; prev = xchg(lock, node)
> + */
> +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> +{
> +	/*
> +	 * We can use relaxed semantics since the caller ensures that the
> +	 * MCS node is properly initialized before updating the tail.
> +	 */
> +	return (u32)xchg_relaxed(&lock->tail,
> +				 tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
> +}
> +
> +#else
>  
>  /**
>   * xchg_tail - Put in the new queue tail code word & retrieve previous one
> @@ -236,7 +240,7 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
>  	}
>  	return old;
>  }
> -#endif /* _Q_PENDING_BITS == 8 */
> +#endif
>  
>  /**
>   * queued_fetch_set_pending_acquire - fetch the whole lock value and set pending
> -- 
> 2.17.1
>