Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace?

Esben Nielsen
2006-02-16T23:24:30Z

On Thu, 16 Feb 2006, Ingo Molnar wrote:

>
> * Esben Nielsen <simlo@phys.au.dk> wrote:
>
> > As I understand the protocol the userspace task writes it's pid into
> > the lock atomically when locking it and erases it atomically when it
> > leaves the lock. If it is killed inbetween the pid is still there. Now
> > if another task comes along it reads the pid, sets the wait flag and
> > goes into the kernel. The kernel will now be able to see that the pid
> > is no longer valid and therefore the owner must be dead.
>
> this is racy - we cannot know whether the PID wrapped around.
>
What about adding more bits to check on? The PID to lookup the task_t and
then some extra bits to uniquely identify the actual task.

> nor does this method offer any solution for the case where there are
> already waiters pending: they might be hung forever.
It was for this case I suggested maintaining a list of waiters within the
kernel on each task_t. The adding has to be done FUTEX_WAIT so the adding
operation needs to be protected.

> With our solution
> one of those waiters gets woken up and notice that the lock is dead.
> (and in the unlikely even of that thread dying too while trying to
> recover the data, the kernel will do yet another wakeup, of the next
> waiter.)
>
I admit your solution is a good one. The only drawback - besides being
untraditional - is that memory corruption can leave futexes locked at
exit.

Esben

> Ingo
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