Re: Change in functionality of futex() system call.

[Darren Hart <dvhart@linux.intel.com>]

On 06/06/2011 10:27 AM, Steven Rostedt wrote:
> On Mon, Jun 06, 2011 at 07:11:37PM +0200, Eric Dumazet wrote:
>> Le lundi 06 juin 2011 à 19:05 +0200, Peter Zijlstra a écrit :
>>
>>> Dunno, using futexes on private file maps is stupid imo, its just asking
>>> for trouble, ro private file maps are even worse. Forcing the COW is the
>>> only sane answer in that it gives consistent results and 'breaks' silly
>>> expectations early instead of sometimes.
>>>
>>> Anyway, that's not really the issue here, as David uses MAP_SHARED (as
>>> one should if one is interested in the shared value).
>>
>> Sure, but maybe another guy is 'stupid' and uses MAP_PRIVATE on its
>> read-only mappings. With old kernels this was working, and we were not
>> doing the COW.
> 
> That sounds like a bug in both the kernel and userspace. I would expect
> a MAP_PRIVATE not be seen by any other process regardless. That's the
> definition of PRIVATE.
> 
>  From: http://www.gnu.org/s/hello/manual/libc/Memory_002dmapped-I_002fO.html
> 
> MAP_PRIVATE
>     This specifies that writes to the region should never be written back
>     to the attached file. Instead, a copy is made for the process, and the
>     region will be swapped normally if memory runs low. No other process
>     will see the changes. 


This doesn't address what happens if changes to a MAP_SHARED mapping are
visible to a MAP_PRIVATE mapping, which is more the issue at hand I believe.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel