linux-kernel - Re: [PATCH 2/9] x86: introduce a set of platform feature flags

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Message-ID: <4A4D41F2.8070507@linux.intel.com>
Date:	Thu, 02 Jul 2009 16:25:38 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	"Pan, Jacob jun" <jacob.jun.pan@...el.com>
CC:	Pavel Machek <pavel@....cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] x86: introduce a set of platform feature flags

Pan, Jacob jun wrote:
>>> +#define platform_has(bit) \
>>> +	test_bit(bit, (unsigned long *)platform_feature)
>> test_bit and friends imply synchronization you probably  don't want or
>> need...
>>
> [[JPAN]] could you explain a little more? The disassembly shows the test_bit
> ends up as a testb instruction and a jump. Why there is synchronization
>  involved?
> Perhaps on some architectures?

test_bit() doesn't imply synchronization; set_bit() and clear_bit() do
(but not __set_bit() and __clear_bit(), just to really be confusing.)

	-hpa
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