linux-kernel - Re: [PATCH 1/5] bitops: add generic implementation of ext2 atomic bitops by test_and_{set,clear}

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Message-ID: <AANLkTik95xrOw0Y_=63EiJ-EGh95XZGeqMy_-HXorqCS@mail.gmail.com>
Date:	Tue, 12 Oct 2010 12:39:37 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/5] bitops: add generic implementation of ext2 atomic
 bitops by test_and_{set,clear}_bit

2010/10/11 Arnd Bergmann <arnd@...db.de>:
> On Sunday 10 October 2010 17:07:26 Akinobu Mita wrote:
>> Some architectures use spinlock to implement it
>> (asm-generic/bitops/ext2-atomic.h). Most other architectures use
>> test_and_set_bit() and test_and_clear_bit() as this patch shows.
>>
>> Why are there two implementations? test_and_{set,clear}_bit() are more
>> costly operations than spinlock for some architectures?
>
> I would guess that is only true on architectures that implement all atomics
> using a hashed spinlock like cris, sparc32 or parisc. This way the user
> can decide which spinlock to use rather than have the arch code calculate
> a hash on the pointer.

Thanks. It looks quite right.

> This is an ext2 specific micro-optimization that I don't think makes
> sense in the generic le bitops code.

OK, I'll only rename ext2 non-atomic bitops to le bitops and
keep ext2 atomic bitops for now.
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