linux-kernel - Re: Regression due to 0c44c2d0f459 x86: Use asm goto to implement better modify_and

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Date:	Wed, 04 Dec 2013 08:44:50 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Regression due to 0c44c2d0f459 x86: Use asm goto to implement
 better modify_and_test() functions

On 12/04/2013 12:59 AM, Ingo Molnar wrote:
> 
> * H. Peter Anvin <hpa@...or.com> wrote:
> 
>> Hi guys,
>>
>> 0c44c2d0f459 x86: Use asm goto to implement better modify_and_test()
>> functions
>>
>> causes a regression, because it incorrectly changed the constraints of
>> bitops.
>>
>> Specifically, the GEN_BINARY_RMWcc() hardcodes a constraint as "er", but
>> it needs to be "Ir" for the bitops themselves.  "I" is correct (as
>> opposed to "J" even on 64 bits, because we only generate the 64-bit
>> version when we have a register operand.
>>
>> Unfortunately there isn't a way we can get gcc+gas to generate a version
>> with an offset pointer.
> 
> Does the regression manifest itself in any actual breakage - if yes, 
> how does it look like? (People experiencing similar symptoms will be 
> helped by seeing a fix matching their problems.)
> 

It was discovered because it caused a build failure in a
not-yet-submitted driver patch.  This happens when someone uses
test_and_set_bit() or another similar operation on a fixed bit index
above 255; the assembler throws an error at that point and the build fails.

*HOWEVER*, for bit indicies in the range 32-255, the current code will
instead silently miscompile, as the CPU will truncate the argument to 5
bits.  I don't know if there are any such instances in the current
kernel, but it is entirely possible there is, with unknown but
potentially disastrous results.

	-hpa

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