Re: [PATCH] checkpoint/restart of robust futex lists

[Oren Laadan <orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org>]

Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org):
>>
>> Matt Helsley wrote:
>>>     Save and restore the [compat_]robust_list member of the task struct.
>>>     
>>>     These lists record which futexes the task holds. To keep the overhead of
>>>     robust futexes low the list is kept in userspace. When the task exits the
>>>     kernel carefully walks these lists to recover held futexes that
>>>     other tasks may be attempting to acquire with FUTEX_WAIT.
>>>     
>>>     Because they point to userspace memory that is saved/restored by
>>>     checkpoint/restart saving the list pointers works.
>>>     
>>>     While saving the pointers works during checkpoint, restart is tricky
>>>     because the robust futex ABI contains provisions for changes based on
>>>     checking the size of the list head. So we need to save the length of
>>>     the list head too in order to make sure that the kernel used during
>>>     restart is capable of handling that ABI. Since there is only one ABI
>>>     supported at the moment taking the list head's size is simple. Should
>>>     the ABI change we will need to use the same size as specified during
>>>     sys_set_robust_list() and hence some new means of determining the length
>>>     of this userspace structure in sys_checkpoint would be required.
>>>     
>>>     Rather than rewrite the logic that checks and handles the ABI we reuse
>>>     sys_set_robust_list() by factoring out the body of the function and
>>>     calling it during restart.
>>>     
>>>     Signed-off-by: Matt Helsley <matthltc-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
>> Patch looks good. Too bad we don't support futex, yet...
> 
> ?
> 
> IIUC (from Matt and Dave), after this patch, you might need something
> for PI futexes, but otherwise non-contended cases "just work" because
> there is no kernel involvement.

That's what I thought. But I also thought that a checkpoint would
fail anyway as soon as it hits the futex-file-descriptor. Or am I
missing something ?

Oren.