On Wed, Feb 28, 2018 at 10:56:32AM +0000, Will Deacon wrote: > On Wed, Feb 28, 2018 at 11:39:32AM +0100, Andrea Parri wrote: > > There appeared to be a certain, recurrent uncertainty concerning the > > semantics of spin_is_locked(), likely a consequence of the fact that > > this semantics remains undocumented or that it has been historically > > linked to the (likewise unclear) semantics of spin_unlock_wait(). > > > > Document this semantics. > > > > Signed-off-by: Andrea Parri <[email protected]> > > Cc: Alan Stern <[email protected]> > > Cc: Will Deacon <[email protected]> > > Cc: Peter Zijlstra <[email protected]> > > Cc: Boqun Feng <[email protected]> > > Cc: Nicholas Piggin <[email protected]> > > Cc: David Howells <[email protected]> > > Cc: Jade Alglave <[email protected]> > > Cc: Luc Maranget <[email protected]> > > Cc: "Paul E. McKenney" <[email protected]> > > Cc: Akira Yokosawa <[email protected]> > > --- > > include/linux/spinlock.h | 11 +++++++++++ > > 1 file changed, 11 insertions(+) > > > > diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h > > index 4894d322d2584..2639fdc9a916c 100644 > > --- a/include/linux/spinlock.h > > +++ b/include/linux/spinlock.h > > @@ -380,6 +380,17 @@ static __always_inline int spin_trylock_irq(spinlock_t > > *lock) > > raw_spin_trylock_irqsave(spinlock_check(lock), flags); \ > > }) > > > > +/** > > + * spin_is_locked() - Check whether a spinlock is locked. > > + * @lock: Pointer to the spinlock. > > + * > > + * This function is NOT required to provide any memory ordering > > + * guarantees; it could be used for debugging purposes or, when > > + * additional synchronization is needed, accompanied with other > > + * constructs (memory barriers) enforcing the synchronization. > > + * > > + * Return: 1, if @lock is (found to be) locked; 0, otherwise. > > + */ > > I also don't think this is quite right, since the spin_is_locked check > must be ordered after all prior lock acquisitions (to any lock) on the same > CPU. That's why we have an smp_mb() in there on arm64 (see 38b850a73034f).
So, arm64 (and powerpc) complies to the semantics I _have_ in mind ... > > So this is a change in semantics and we need to audit the users before > proceeding. We should also keep spin_is_locked consistent with the versions > for mutex, rwsem, bit_spin. Well, strictly speaking, it isn't (given that the current semantics is, as reported above, currently undocumented); for the same reason, cases relying on anything more than _nothing_ (if any) are already broken ... Andrea > > Will
