----- Original Message -----
> Hi Jan, Waiman, [+Jade and Paul for the litmus test at the end]
>
> On Wed, Jul 17, 2019 at 09:22:00PM +0200, Jan Stancek wrote:
> > On Wed, Jul 17, 2019 at 10:19:04AM -0400, Waiman Long wrote:
> > > > If you add a comment to the code outlining the issue (preferably as a
> > > > litmus
> > > > test involving sem->count and some shared data which happens to be
> > > > vmacache_seqnum in your test)), then:
> > > >
> > > > Reviewed-by: Will Deacon <w...@kernel.org>
> > > >
> > > > Thanks,
> > > >
> > > > Will
> > >
> > > Agreed. A comment just above smp_acquire__after_ctrl_dep() on why this
> > > is needed will be great.
> > >
> > > Other than that,
> > >
> > > Acked-by: Waiman Long <long...@redhat.com>
> > >
> >
> > litmus test looks a bit long, would following be acceptable?
> >
> > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> > index 37524a47f002..d9c96651bfc7 100644
> > --- a/kernel/locking/rwsem.c
> > +++ b/kernel/locking/rwsem.c
> > @@ -1032,6 +1032,13 @@ static inline bool rwsem_reader_phase_trylock(struct
> > rw_semaphore *sem,
> > */
> > if (adjustment && !(atomic_long_read(&sem->count) &
> > (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> > + /*
> > + * down_read() issued ACQUIRE on enter, but we can race
> > + * with writer who did RELEASE only after us.
> > + * ACQUIRE here makes sure reader operations happen only
> > + * after all writer ones.
> > + */
>
> How about an abridged form of the litmus test here, just to show the cod
> flow? e.g.:
>
> /*
> * 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>
> */
Works for me. The code is at 3 level of indentation, but I can try
to squeeze it in for v4.
>
> In writing this, I also noticed that we don't have any explicit ordering
> at the end of the reader slowpath when we wait on the queue but get woken
> immediately:
>
> if (!waiter.task)
> break;
>
> Am I missing something?
I'm assuming this isn't problem, because set_current_state() on line above
is using smp_store_mb().
>
> > + smp_acquire__after_ctrl_dep();
> > raw_spin_unlock_irq(&sem->wait_lock);
> > rwsem_set_reader_owned(sem);
> > lockevent_inc(rwsem_rlock_fast);
> >
> >
> > with litmus test in commit log:
> > ----------------------------------- 8< ------------------------------------
> > C rwsem
> >
> > {
> > atomic_t rwsem_count = ATOMIC_INIT(1);
> > int vmacache_seqnum = 10;
> > }
> >
> > P0(int *vmacache_seqnum, atomic_t *rwsem_count)
> > {
> > r0 = READ_ONCE(*vmacache_seqnum);
> > WRITE_ONCE(*vmacache_seqnum, r0 + 1);
> > /* downgrade_write */
> > r1 = atomic_fetch_add_release(-1+256, rwsem_count);
> > }
> >
> > P1(int *vmacache_seqnum, atomic_t *rwsem_count, spinlock_t *sem_wait_lock)
> > {
> > /* rwsem_read_trylock */
> > r0 = atomic_add_return_acquire(256, rwsem_count);
> > /* rwsem_down_read_slowpath */
> > spin_lock(sem_wait_lock);
> > r0 = atomic_read(rwsem_count);
> > if ((r0 & 1) == 0) {
> > // BUG: needs barrier
> > spin_unlock(sem_wait_lock);
> > r1 = READ_ONCE(*vmacache_seqnum);
> > }
> > }
> > exists (1:r1=10)
> > ----------------------------------- 8< ------------------------------------
>
> Thanks for writing this! It's definitely worth sticking it in the commit
> log, but Paul and Jade might also like to include it as part of their litmus
> test repository too.
>
> Will
>
