Re: PI BUG with -rt13

From: david singleton
Date: Fri Nov 18 2005 - 12:05:04 EST

On Nov 18, 2005, at 5:27 AM, Ingo Molnar wrote:

* Dinakar Guniguntala <dino@xxxxxxxxxx> wrote:

Ingo, Thanks for providing the fix. However I still hit the same bug
even with your changes

even with my patch the robust-futex code is still quite broken. E.g. in
down_futex(), it accesses rt-mutex internals without any locking (!):

if (rt_mutex_free(lock)) {
__down_mutex(lock __EIP__);
rt_mutex_set_owner(lock, owner_task->thread_info);

both rt_mutex_free() and rt_mutex_set_owner() must be called with the
proper locking. David?

Yes, the lock needs to be protected by the robust semaphore.
The locking order is:

mmap_sem to protect the vma that holds the pthread_mutex

robust_sem to protect the futex_mutex chain.

futex_mutex - the rt_mutex associated with the pthread_mutex.

This section of code performs the locking of the first waiter
on the pthread_mutex, which was locked by another thread in the
fast path in userspace. The fast path locking does not enter the

The first waiter locks the futex_mutex and then changes the
owner to the 'owning' thread. The waiter then calls down_interruptible
to block on the futex_mutex.

I'll provide a patch to protect this piece of code.



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