Re: [PATCH 10/11] x86, rwsem: provide __down_write_killable

[H. Peter Anvin]

On April 20, 2016 6:40:19 AM PDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>On Wed, Apr 13, 2016 at 02:49:43PM +0200, Michal Hocko wrote:
>> On Wed 13-04-16 12:27:31, Ingo Molnar wrote:
>> > 
>> > * Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>> > 
>> > > I'm testing your patches today, if they are otherwise OK [...]
>> > 
>> > got this build failure:
>> > 
>> >   ./arch/x86/include/asm/rwsem.h:106:2: error: âasmâ operand has
>impossible constraints
>> 
>> Hmm, I have no idea why 64b didn't have problem with the asm but 32b
>> complains. Anyway, the following makes both happy. I have checked the
>> generated code for 64b and it hasn't changed after the patch. 32b
>also
>> seems to be generating a proper code. My gcc asm()-foo is rather weak
>so
>> I would feel better if somebody double checked after me.
>
>I completely blow at this gcc-asm constraints thing too :/
>
>In any case, Ingo will you look after the rest of these patches, or do
>you want me to pick up the remaining bits?

The reason it breaks is because the same register can't be an input-output register and a separate input.  However, the input side of the input-output is probably undefined, and so gcc may not notice.
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