Messages in this thread | | | Date | Thu, 13 Apr 2017 19:59:07 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work_done() |
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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).
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