On Thu, Apr 13, 2017 at 07:51:36PM +0200, Peter Zijlstra wrote:
> > I suppose that one alternative is the new variant of kerneldoc, though
> > very few of these functions have comment headers, let alone kerneldoc
> > headers. Which reminds me, the question of spin_unlock_wait() and
> > spin_is_locked() semantics came up a bit ago. Here is what I believe
> > to be the case. Does this match others' expectations?
> >
> > o spin_unlock_wait() semantics:
> >
> > 1. Any access in any critical section prior to the
> > spin_unlock_wait() is visible to all code following
> > (in program order) the spin_unlock_wait().
> >
> > 2. Any access prior (in program order) to the
> > spin_unlock_wait() is visible to any critical
> > section following the spin_unlock_wait().
> >
> > o spin_is_locked() semantics: Half of spin_unlock_wait(),
> > but only if it returns false:
> >
> > 1. Any access in any critical section prior to the
> > spin_unlock_wait() is visible to all code following
> > (in program order) the spin_unlock_wait().
>
> Urgh.. yes those are pain. The best advise is to not use them.
>
> 055ce0fd1b86 ("locking/qspinlock: Add comments")
The big problem with spin_unlock_wait(), aside from the icky barrier
semantics, is that it tends to end up prone to starvation. So where
spin_lock()+spin_unlock() have guaranteed fwd progress if the lock is
fair (ticket,queued,etc..) spin_unlock_wait() must often lack that
guarantee.
Equally, spin_unlock_wait() was intended to be 'cheap' and be a
read-only loop, but in order to satisfy the barrier requirements, it
ends up doing stores anyway (see for example the arm64 and ppc
implementations).
