On Thu, Jul 18, 2019 at 06:50:52AM -0400, Jan Stancek wrote: > > ----- 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 <[email protected]> > > > > > > > > > > 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 <[email protected]> > > > > > > > > > > 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.
Thank you for forwarding this! It may now be found at: https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-JanStancek-rwsem.litmus Thanx, Paul
