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

[Esben Nielsen <simlo@phys.au.dk>]

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 is no longer valid and therefore the owner must be dead.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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