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Date: Mon, 16 Nov 2015 16:56:58 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
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        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
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On Thu, Nov 12, 2015 at 10:21:39AM -0800, Linus Torvalds wrote:
> Now, the point of spin_unlock_wait() (and "spin_is_locked()") should
> generally be that you have some external ordering guarantee that
> guarantees that the lock has been taken. For example, for the IPC
> semaphores, we do either one of:
> 
>  (a) get large lock, then - once you hold that lock - wait for each small lock
> 
> or
> 
>  (b) get small lock, then - once you hold that lock - check that the
> largo lock is unlocked
> 
> and that's the case we should really worry about.  The other uses of
> spin_unlock_wait() should have similar "I have other reasons to know
> I've seen that the lock was taken, or will never be taken after this
> because XYZ".

I don't think this is true for the usage in do_exit(), we have no
knowledge on if pi_lock is taken or not. We just want to make sure that
_if_ it were taken, we wait until it is released.

But I'm not sure where task_work_run() sits, at first reading it appears
to also not be true -- there doesn't appear to be a reason we know a
lock to be held.

It does however appear true for the usage in completion_done(), where by
having tested x->done, we know a pi_lock _was_ held.