Messages in this thread | | | Date | Thu, 20 Apr 2017 17:08:26 +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 20, 2017 at 08:03:21AM -0700, Paul E. McKenney wrote: > On Thu, Apr 20, 2017 at 01:17:43PM +0200, Peter Zijlstra wrote:
> > > +/** > > > + * spin_is_locked - Conditionally interpose after prior critical sections > > > + * @lock: the spinlock whose critical sections are to be interposed. > > > + * > > > + * Semantically this is equivalent to a spin_trylock(), and, if > > > + * the spin_trylock() succeeds, immediately followed by a (mythical) > > > + * spin_unlock_relaxed(). The return value from spin_trylock() is returned > > > + * by spin_is_locked(). Note that all current architectures have extremely > > > + * efficient implementations in which the spin_is_locked() does not even > > > + * write to the lock variable. > > > + * > > > + * A successful spin_is_locked() primitive in some sense "takes its place" > > > + * after some critical section for the lock in question. Any accesses > > > + * following a successful spin_is_locked() call will therefore happen > > > + * after any accesses by any of the preceding critical section for that > > > + * same lock. Note however, that spin_is_locked() provides absolutely no > > > + * ordering guarantees for code preceding the call to that spin_is_locked(). > > > + */ > > > static __always_inline int spin_is_locked(spinlock_t *lock) > > > { > > > return raw_spin_is_locked(&lock->rlock); > > > > I'm current confused on this one. The case listed in the qspinlock code > > doesn't appear to exist in the kernel anymore (or at least, I'm having > > trouble finding it). > > > > That said, I'm also not sure spin_is_locked() provides an acquire, as > > that comment has an explicit smp_acquire__after_ctrl_dep(); > > OK, I have dropped this portion of the patch for the moment. > > Going forward, exactly what semantics do you believe spin_is_locked() > provides? > > Do any of the current implementations need to change to provide the > semantics expected by the various use cases?
I don't have anything other than the comment I wrote back then. I would have to go audit all spin_is_locked() implementations and users (again).
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