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

[Oren Laadan]

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...

Oren.


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