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 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. Will
