Messages in this thread | | | Date | Fri, 17 Feb 2006 01:20:18 +0100 (MET) | From | Esben Nielsen <> | Subject | Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace? |
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On Fri, 17 Feb 2006, Ingo Molnar wrote:
> > * Esben Nielsen <simlo@phys.au.dk> wrote: > > > > 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. > > which would just be a fancy name for a wider PID space, and would thus > still not protect against PID reuse :-) > Can it really be correct there is no way to uniquely identify a thread in the uptime of the system? It could be done with BigIntegers :-)
> > > 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. > > i'm not sure i follow - what list is this and how would it be > maintained? >
At the FUTEX_WAIT operation add the waiter to a list of waiters on the owner's task_t. At FUTEX_WAKE remove the waiter. At task exit wake up the waiters.
> > > 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. > > so? Memory corruption can overwrite the futex value anyway, and can thus > cause the wrong owner to be identified - causing a locked futex. This > patch does not protect against bad effects of memory corruption - > there's really no way to keep userspace from breaking itself. >
At least you could wake up those who are already blocked in the kernel...
Esben
> Ingo >
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