linux-kernel - Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian

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Message-ID: <20160608092244.GA27029@arm.com>
Date:	Wed, 8 Jun 2016 10:22:44 +0100
From:	Will Deacon <will.deacon@....com>
To:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	arnd@...db.de, waiman.long@...com, peterz@...radead.org
Subject: Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian

On Thu, Jun 02, 2016 at 06:09:08PM +0800, Pan Xinhui wrote:
> strcut __qrwlock has different layout in big endian machine. we need set
> the __qrwlock->wmode to NULL, and the address is not &lock->cnts in big
> endian machine.
> 
> Do as what read unlock does. we are lucky that the __qrwlock->wmode's
> val is _QW_LOCKED.

Doesn't this have wider implications for the qrwlocks, for example:

  while ((cnts & _QW_WMASK) == _QW_LOCKED) { ... }

would actually end up looking at the wrong field of the lock?

Shouldn't we just remove the #ifdef __LITTLE_ENDIAN stuff from __qrwlock,
given that all the struct members are u8?

Will