Re: robust futex deadlock detection patch

[Esben Nielsen]

On Mon, 9 Jan 2006, Steven Rostedt wrote:

> On Mon, 2006-01-09 at 21:16 +0100, Esben Nielsen wrote:
>
> > You only take the spinlocks corresponding to the current lock. What about
> > the next locks in the chain? Remember those locks might not be
> > futexes but a lock inside the kernel, taken in system calls. I.e. the
> > robust_sem might not protect you.
> > If there are n locks you need to lock n lock->wait_lock and n
> > owner->task->pi_lock as you traverse the locks. That is what I tried to
> > sketch in the examble below.
>
> The thing about this is that you can't (if the kernel is not buggy)
> deadlock on the kernel spin locks.  As long as you protect the locks in
> the futex from deadlocking you are fine.  This is because you don't grab
> a futex after grabbing a kernel spin lock.  All kernel spin locks
> __must__ be released before going back to user land, and that's where
> you grab the futexes.
>
Yes, but the deadlock-detect code doesn't know what is a futex and what
isn't. Therefore when it starts to traverse rt_mutexes which aren't
futexes it has to be _very_ carefull taking the right spinlocks. On the
other hand when traversing the futexes it has to be carefull not to
deadlocks on those very same spinlocks.

And yes, you _can_ traverse the lock chain without deadlocking the kernel.
The code I sketched in the previouse mail should do it.

Anyway, I am trying to rewrite rt_mutex to remove the spinlock deadlock
altogether. My girlfriend is away with our daugther a few days so I might
get time to do finish it :-)

> -- Steve
>
>

Esben

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