Re: sem_lock() vs qspinlocks

[Davidlohr Bueso <dave@stgolabs.net>]

On Fri, 20 May 2016, Peter Zijlstra wrote:

>The problem is that the clear_pending_set_locked() is an unordered
>store, therefore this store can be delayed until no later than
>spin_unlock() (which orders against it due to the address dependency).
>
>This opens numerous races; for example:
>
>	ipc_lock_object(&sma->sem_perm);
>	sem_wait_array(sma);
>
>				false   ->	spin_is_locked(&sma->sem_perm.lock)
>
>is entirely possible, because sem_wait_array() consists of pure reads,
>so the store can pass all that, even on x86.

I had pondered at the unordered stores in clear_pending_set_locked() for arm,
for example, but I _certainly_ missed this for x86 inside the ACQUIRE region.

Thanks,
Davidlohr