On Thu, Jul 18, 2019 at 12:45:47PM +0100, Will Deacon wrote:
> On Thu, Jul 18, 2019 at 12:58:12PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 18, 2019 at 10:26:41AM +0100, Will Deacon wrote:
> >
> > > /*
> > > * We need to ensure ACQUIRE semantics when reading sem->count so that
> > > * we pair with the RELEASE store performed by an unlocking/downgrading
> > > * writer.
> > > *
> > > * P0 (writer) P1 (reader)
> > > *
> > > * down_write(sem);
> > > * <write shared data>
> > > * downgrade_write(sem);
> > > * -> fetch_add_release(&sem->count)
> > > *
> > > * down_read_slowpath(sem);
> > > * -> atomic_read(&sem->count)
> > > * <ctrl dep>
> > > * smp_acquire__after_ctrl_dep()
> > > * <read shared data>
> > > */
> >
> > So I'm thinking all this is excessive; the simple rule is: lock acquire
> > should imply ACQUIRE, we all know why.
>
> Fair enough, I just thought this was worth highlighting because you can't
> reply on the wait_lock to give you ACQUIRE ordering.
Right, not in this case, because sem->count is not fully serialized by
it, whereas below the wait-queue is.
> > ---
> > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> > index 37524a47f002..9eb630904a17 100644
> > --- a/kernel/locking/rwsem.c
> > +++ b/kernel/locking/rwsem.c
> > @@ -1000,6 +1000,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem,
> > int state)
> > atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> > adjustment = 0;
> > if (rwsem_optimistic_spin(sem, false)) {
> > + /* rwsem_optimistic_spin() implies ACQUIRE through
> > rwsem_*trylock() */
>
> I couldn't figure out if this was dependent on the return value or not,
I went with the fact that the only way to return true is if taken
becomes true; and that only happens through
rwsem_try_{read,write}_lock_unqueued(), and both imply ACQUIRE on
success.
> and looking at osq_lock() I also couldn't see the ACQUIRE barrier when we're
> spinning on node->locked. Hmm.
Yes, osq is not a full lock and does not imply these barriers. This came
up somewhere, did we forget to write a comment on that? Lemme go look.
> > /*
> > * Wake up other readers in the wait list if the front
> > * waiter is a reader.
> > @@ -1014,6 +1015,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem,
> > int state)
> > }
> > return sem;
> > } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
> > + /* rwsem_reader_phase_trylock() implies ACQUIRE */
>
> Can we add "on success" to the end of this, please?
Good point.
> > return sem;
> > }
> >
> > @@ -1032,6 +1034,8 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem,
> > int state)
> > */
> > if (adjustment && !(atomic_long_read(&sem->count) &
> > (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> > + /* Provide lock ACQUIRE */
> > + smp_acquire__after_ctrl_dep();
> > raw_spin_unlock_irq(&sem->wait_lock);
> > rwsem_set_reader_owned(sem);
> > lockevent_inc(rwsem_rlock_fast);
> > @@ -1065,15 +1069,25 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem,
> > int state)
> > wake_up_q(&wake_q);
> >
> > /* wait to be given the lock */
> > - while (true) {
> > + for (;;) {
> > set_current_state(state);
> > - if (!waiter.task)
> > + if (!smp_load_acquire(&waiter.task)) {
> > + /*
> > + * Matches rwsem_mark_wake()'s smp_store_release() and
> > ensures
> > + * we're ordered against its sem->count operations.
> > + */
> > break;
> > + }
>
> Ack. Also, grepping for 'waiter.task' reveals a similar usage in
> drivers/tty/tty_ldsem.c if you're feeling brave enough.
*sigh* of course, for every bug there needs to be a second copy
somewhere.
I'll go look there too. Thanks!
