linux-kernel - Re: [PATCH v2] locking/qrwlock: Let qrwlock has same layout regardless of the endian

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Date:	Fri, 17 Jun 2016 15:06:33 -0400
From:	Waiman Long <waiman.long@....com>
To:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
	<ingo@...hat.com>, <peterz@...radead.org>, <arnd@...db.de>,
	<will.deacon@....com>
Subject: Re: [PATCH v2] locking/qrwlock: Let qrwlock has same layout regardless
 of the endian

On 06/15/2016 05:31 AM, Pan Xinhui wrote:
> This patch aims to get rid of endianness in queued_write_unlock(). We
> want to set  __qrwlock->wmode to NULL, however the address is not
> &lock->cnts in big endian machine. That causes queued_write_unlock()
> write NULL to the wrong field of __qrwlock.
>
> Actually qrwlock can have same layout, IOW we can remove the #if
> __little_endian in struct __qrwlock. With such modification, we only
> need define some _QW* and _QR* with corresponding values in different
> endian systems.
>
> Suggested-by: Will Deacon<will.deacon@....com>
> Signed-off-by: Pan Xinhui<xinhui.pan@...ux.vnet.ibm.com>
> ---
> change from v1:
> 	A typo fix which is really bad...
> 	thanks Will for the carefull review. :)
> ---
>   include/asm-generic/qrwlock.h | 15 +++++++++++----
>   kernel/locking/qrwlock.c      | 10 ++++------
>   2 files changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 54a8e65..28fb94a 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -27,11 +27,18 @@
>   /*
>    * Writer states&  reader shift and bias
>    */
> -#define	_QW_WAITING	1		/* A writer is waiting	   */
> -#define	_QW_LOCKED	0xff		/* A writer holds the lock */
> -#define	_QW_WMASK	0xff		/* Writer mask		   */
> +#ifdef	__LITTLE_ENDIAN
>   #define	_QR_SHIFT	8		/* Reader count shift	   */
> -#define _QR_BIAS	(1U<<  _QR_SHIFT)
> +#define	_QW_SHIFT	0		/* Writer mode shift	*/
> +#else
> +#define	_QR_SHIFT	0		/* Reader count shift	   */
> +#define	_QW_SHIFT	24		/* Writer mode shift	*/
> +#endif
> +
> +#define	_QW_WAITING	(1U<<  _QW_SHIFT)	/* A writer is waiting	   */
> +#define	_QW_LOCKED	(0xffU<<  _QW_SHIFT)	/* A writer holds the lock */
> +#define	_QW_WMASK	(0xffU<<  _QW_SHIFT)	/* Writer mask		   */
> +#define	_QR_BIAS	(1U<<  _QR_SHIFT)
>
>   /*
>    * External function declarations
> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index fec0823..57d66cf 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -30,18 +30,15 @@ struct __qrwlock {
>   	union {
>   		atomic_t cnts;
>   		struct {
> -#ifdef __LITTLE_ENDIAN
>   			u8 wmode;	/* Writer mode   */
>   			u8 rcnts[3];	/* Reader counts */
> -#else
> -			u8 rcnts[3];	/* Reader counts */
> -			u8 wmode;	/* Writer mode   */
> -#endif
>   		};
>   	};
>   	arch_spinlock_t	lock;
>   };
>
> +#define	_QW_MODEVAL(v)	((v)>>  _QW_SHIFT)

I know what you are doing here, but it is a bit hard to understand it 
just by looking at the name of the macro itself. Maybe some other names 
like _QW_MASKVAL() or_QW_BYTEVAL(). You may also want to have a line of 
comment about it. Other than that, I don't see any problem with it.

Acked-by: Waiman Long <Waiman.Long@....com>



> +
>   /**
>    * rspin_until_writer_unlock - inc reader count&  spin until writer is gone
>    * @lock  : Pointer to queue rwlock structure
> @@ -127,7 +124,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
>   		struct __qrwlock *l = (struct __qrwlock *)lock;
>
>   		if (!READ_ONCE(l->wmode)&&
> -		   (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
> +		   (cmpxchg_relaxed(&l->wmode, 0,
> +					_QW_MODEVAL(_QW_WAITING)) == 0))
>   			break;
>
>   		cpu_relax_lowlatency();