Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty

[Jan Stancek <jstancek@redhat.com>]

----- 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 <will@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 <longman@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
>