[PATCH v5.2 2/2] lcoking/rwsem: Add missing ACQUIRE to read_slowpath sleep loop

[Sasha Levin]

From: Peter Zijlstra <pet...@infradead.org>

[ Upstream commit 99143f82a255e7f054bead8443462fae76dd829e ]

While reviewing another read_slowpath patch, both Will and I noticed
another missing ACQUIRE, namely:

  X = 0;

  CPU0                  CPU1

  rwsem_down_read()
    for (;;) {
      set_current_state(TASK_UNINTERRUPTIBLE);

                        X = 1;
                        rwsem_up_write();
                          rwsem_mark_wake()
                            atomic_long_add(adjustment, &sem->count);
                            smp_store_release(&waiter->task, NULL);

      if (!waiter.task)
        break;

      ...
    }

  r = X;

Allows 'r == 0'.

Reported-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Reported-by: Will Deacon <w...@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Acked-by: Will Deacon <w...@kernel.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Sasha Levin <sas...@kernel.org>
---

This is a backport for the v5.2 stable tree. There were multiple reports  
of this issue being hit.

Given that there were a few changes to the code around this, I'd
appreciate an ack before pulling it in.

 kernel/locking/rwsem-xadd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 397dedc58432d..385ebcfc31a6d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -485,8 +485,10 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, 
int state)
        /* wait to be given the lock */
        while (true) {
                set_current_state(state);
-               if (!waiter.task)
+               if (!smp_load_acquire(&waiter.task)) {
+                       /* Orders against rwsem_mark_wake()'s 
smp_store_release() */
                        break;
+               }
                if (signal_pending_state(state, current)) {
                        raw_spin_lock_irq(&sem->wait_lock);
                        if (waiter.task)
-- 
2.20.1