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

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
> 
> 
> 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 at us.ibm.com>
> 
> 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.

-serge
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