Re: [RFC -tip 2/4] x86: uaccess: introduce __{get|put}_user exception handling framework

[Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>]

Ingo Molnar wrote:
> * Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com> wrote:
> 
>> From: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
>>
>> Impact: introduce new framework
>>
>> Introduce exception handling framework.
>> __{get|put}_user_ex_try() begins exception block and
>> __{get|put}_user_ex_catch() ends block and if an exception occurred in this
>> block using __{get|put}_user_ex, direct jump to __{get|put}_user_ex_catch()
>> and err is set to specified value.
> 
> ha, this tickled ~12 year old memories: back then Linus came up with a 
> very, very similar scheme, for user-copy exception handling.
> 
> Such a scheme would be elegant, creates more compact code (we can use 
> conditional results directly in branch instructions instead of having to 
> export them into registers), and it makes sense syntactically, but it 
> doesnt work: GCC is free to reorder (or eliminate) basic blocks and these 
> labels can lose their relationship.
> 
> So this cannot be done via inline assembly right now, it needs some 
> compiler help. Sniff :)

thanks for the above explanation.
I felt it's very hard to understand GCC.

Thanks,
Hiroshi