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Re: Futex error handling

On Thu, Sep 18, 2014 at 02:26:35PM +0200, Torvald Riegel wrote:
> > The EFAULT case with
> > FUTEX_WAKE, and which I claim FUTEX_WAKE_OP avoids, is when the atomic
> > operation on the futex int that's associated with the wake allows
> > another thread to synchronize and determine that it may legally
> > destroy the object before the actual wake is sent. FUTEX_WAKE_OP can
> > fully avoid this by performing the atomic operation after looking up
> > and locking the futex hash bucket, so that there's no further access
> > after the atomic and thus no opportunity for fault.
> Agreed; that like what UNLOCK_PI does.  However, and that's something
> I've only thought about recently, it would be good to know which
> guarantees the kernel gives in this case; in particular, what happens
> (and which error code results) if there is destruction and potential
> unmapping etc. of the futex variable concurrently with WAKE_OP or
> UNLOCK_PI being in flight.

I've RTFS'd and my understanding is that no such problems are
possible. The futex hashing (note: there are two futex address
arguments and both are hashed, even if they're equal; this should be
optimized on the kernel side to make FUTEX_WAKE_OP practical) and
locking of the resulting hash buckets happens before the atomic
operation is performed. After the atomic operation, the bucket is
walked and matching waiters are woken.

In theory it's possible that, as soon as the atomic operation is
performed, the backing (file/anon/whatever) is destroyed and its
underlying id (e.g. inode number) is reused, so that the backing
identified for the original futex address has been reused by this
time. However, it's not a problem because a new waiter can't arrive
while the hash bucket is still locked -- so such a new waiter can't be


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