Re: [PATCH v4 08/16] locking/rwsem: Make rwsem_spin_on_owner() return owner state

From: Peter Zijlstra
Date: Wed Apr 17 2019 - 09:00:29 EST


On Wed, Apr 17, 2019 at 02:41:01PM +0200, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:51PM -0400, Waiman Long wrote:
> > In the special case that there is no active lock and the handoff bit
> > is set, optimistic spinning has to be stopped.
>
> > @@ -500,9 +521,19 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
> >
> > /*
> > * If there is a new owner or the owner is not set, we continue
> > - * spinning.
> > + * spinning except when here is no active locks and the handoff bit
> > + * is set. In this case, we have to stop spinning.
> > */
> > - return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
> > + owner = READ_ONCE(sem->owner);
> > + if (!is_rwsem_owner_spinnable(owner))
> > + return OWNER_NONSPINNABLE;
> > + if (owner && !is_rwsem_owner_reader(owner))
> > + return OWNER_WRITER;
> > +
> > + count = atomic_long_read(&sem->count);
> > + if (RWSEM_COUNT_HANDOFF(count) && !RWSEM_COUNT_LOCKED(count))
> > + return OWNER_NONSPINNABLE;
> > + return !owner ? OWNER_NULL : OWNER_READER;
> > }
>
> So this fixes a straight up bug in the last patch (and thus should be
> done before so the bug never exists), and creates unreadable code while
> at it.
>
> Also, I think only checking HANDOFF after the loop is wrong; the moment
> HANDOFF happens you have to terminate the loop, irrespective of what
> @owner does.
>
> Does something like so work?
>
> ---

enum owner_state {
OWNER_NULL = 1 << 0,
OWNER_WRITER = 1 << 1,
OWNER_READER = 1 << 2,
OWNER_NONSPINNABLE = 1 << 3,
};
#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER)

static inline enum owner_state rwsem_owner_state(unsigned long owner)
{
if (!owner)
return OWNER_NULL;

if (owner & RWSEM_ANONYMOUSLY_OWNED)
return OWNER_NONSPINNABLE;

if (owner & RWSEM_READER_OWNER)
return OWNER_READER;

return OWNER_WRITER;
}

static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *tmp, *owner = READ_ONCE(sem->owner);
enum owner_state state = rwsem_owner_state((unsigned long)owner);

if (state != OWNER_WRITER)
return state;

rcu_read_lock();
for (;;) {
if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) {
state = OWNER_NONSPINNABLE;
break;
}

tmp = READ_ONCE(sem->owner);
if (tmp != owner) {
state = rwsem_owner_state((unsigned long)tmp);
break;
}

/*
* Ensure we emit the owner->on_cpu, dereference _after_
* checking sem->owner still matches owner, if that fails,
* owner might point to free()d memory, if it still matches,
* the rcu_read_lock() ensures the memory stays valid.
*/
barrier();

if (need_resched() || !owner_on_cpu(owner)) {
state = OWNER_NONSPINNABLE;
break;
}

cpu_relax();
}
rcu_read_unlock();

return state;
}