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

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Date:	Tue, 14 Jun 2016 14:11:48 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....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 2016年06月08日 17:22, Will Deacon wrote:
> 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?
>
I does not clearly understand your idea. :(
the condition in the while() is always true from the view of current code.
BUT if __qrwlock has same layout on the two endian machine, the while() will end up. :)


> Shouldn't we just remove the #ifdef __LITTLE_ENDIAN stuff from __qrwlock,
> given that all the struct members are u8?
>
No. that makes codes complex. for example

struct __qrwlock lock;

WRITE_ONCE(lock->wmode, _QW_WAITING);
if (atomic_(&lock->cnts) == _QW_WAITING) {
	do_something();
}

IF you remove the  #ifdef __LITTLE_ENDIAN stuff from __qrwlock.
codes above obviously will break. And we already have such code.

thanks
xinhui

> Will
>