linux-kernel - Re: [PATCH v3] locking/qrwlock: Let qrwlock has same layout regardless of the endian

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Date:	Wed, 13 Jul 2016 21:54:46 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	<linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
	<mingo@...hat.com>, <arnd@...db.de>
Subject: Re: [PATCH v3] locking/qrwlock: Let qrwlock has same layout regardless
 of the endian

On 07/13/2016 03:54 PM, Peter Zijlstra wrote:
> On Mon, Jun 20, 2016 at 02:20:52PM +0800, 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>
>> Acked-by: Waiman Long<Waiman.Long@....com>
>> ---
> Urgh, I hate this stuff :/
>
> OK, so I poked at this a bit and I ended up with the below; but now
> qrwlock and qspinlock are inconsistent; although I suspect qspinlock is
> similarly busted wrt endian muck.

Qspinlock is a bit different that the generic unlock code is 
endian-independent. The x86 architectural unlock code, however, is 
little-endian specific. This is not a problem. PPC will certainly needs 
its own unlock code for better performance.

With this patch, there is indeed inconsistency on how qspinlock and 
qrwlock handle their internal data. I guess we could make qrwlock more 
like qspinlock if you want consistency, but I am fine with either ways.

> Not sure what to do..
>
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -25,13 +25,31 @@
>   #include<asm-generic/qrwlock_types.h>
>
>   /*
> - * Writer states&  reader shift and bias
> + * Writer states&  reader shift and bias.
> + *
> + *       | +0 | +1 | +2 | +3 |
> + *   ----+----+----+----+----+
> + *    LE | 12 | 34 | 56 | 78 | 0x12345678
> + *   ----+----+----+----+----+
> + *    BE | 78 | 56 | 34 | 12 | 0x12345678
> + *   ----+----+----+----+----+
> + *       | wr |      rd      |
> + *       +----+----+----+----+
> + *
>    */
> -#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	(0x01U<<  _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	(0x01U<<  _QR_SHIFT)
>   

I like the comment at the top (after you fixed the typo :-) ).

>   /*
>    * External function declarations
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -22,26 +22,6 @@
>   #include<linux/hardirq.h>
>   #include<asm/qrwlock.h>
>
> -/*
> - * This internal data structure is used for optimizing access to some of
> - * the subfields within the atomic_t cnts.
> - */
> -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;
> -};
> -
>   /**
>    * rspin_until_writer_unlock - inc reader count&  spin until writer is gone
>    * @lock  : Pointer to queue rwlock structure
> @@ -124,10 +104,10 @@ void queued_write_lock_slowpath(struct q
>   	 * or wait for a previous writer to go away.
>   	 */
>   	for (;;) {
> -		struct __qrwlock *l = (struct __qrwlock *)lock;
> +		u8 *wr = (u8 *)lock;
>
> -		if (!READ_ONCE(l->wmode)&&
> -		   (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
> +		if (!READ_ONCE(*wr)&&
> +		    (cmpxchg_relaxed(wr, 0, _QW_WAITING>>  _QW_SHIFT) == 0))
>   			break;
>
>   		cpu_relax_lowlatency();

Cheers,
Longman