Re: ERESTARTSYS escaping from sem_wait with RTLinux patch

[Darren Hart]

Blaise Gassend wrote:
> > When 3495 finally get's to run and complete it's futex_wake() call, the task
> > still needs to be woken, so we wake it - but now it's enqueued with a different
> > futex_q, which now has a valid lock_ptr, so upon wake-up we expect a signal!
> >
> > OK, I believe this establishes root cause.  Now to come up with a fix...
>
> Wow, good work Darren! You definitely have more experience than I do at
> tracking down these in-kernel races, and I'm glad we have you looking at
> this. I'm snarfing up useful techniques from your progress emails.

Great, I learn a lot from reading other people's status-type email as 
well.  Glad I can be on the contributing end once and a while :)

> So if I understand correctly, there is a race between wake_futex and a
> timeout (or a signal, but AFAIK when my python example is running
> steady-state there are no signals). The problem occurs when wake_futex
> gets preempted half-way through, after it has NULLed lock_ptr, but
> before it has woken up the waiter. If a timeout/signal wakes the waiter,
> and the waiter runs and starts waiting again before the waker resumes,
> then the waker will end up waking the waiter a second time, without the
> lock_ptr for the second wait being NULLified. This causes the waiter to
> mis-interpret what woke it and leads to the fault we observed.
>
> Now I am wondering if the workaround described here
> http://markmail.org/message/at2s337yz3wclaif
> for what seems like this same problem isn't actually a legitimate fix.
> It ends up looking something like this: (lines 3 and 4 are new)
>
>   ret = -ERESTARTSYS;
>   if (!abs_time) {
>     if (!signal_pending(current))
>       set_tsk_thread_flag(current,TIF_SIGPENDING);
>     goto out_put_key;
>   }

The trouble with this is it is a bandaid to a fundamentally broken 
wake-up path. I tried flagging the waiters on the wake-list as already 
woken and then skipping them in the wake_futex_list(), but this got ugly 
really fast.

Talking with Thomas a bit more we're not sure the patch that introduced 
this lockless waking actually does any good, as the normal wakeup path 
doesn't take the hb->lock anyway, it's more likely the contention was 
due to an app like this that wakes a task and almost immediately puts it 
back to sleep on a futex before the waker has a chance to drop the hb->lock.

The futex wake-up path is complicated enough as it is, in my personal 
opinion, we are better off dropping the "lockless wake-up" patch and 
removing the race and simplifying the wake-up path at the same time.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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