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

[Jan Stancek]

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.
+                        */
+                       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,
Jan