linux-kernel - [PATCH 2/4] rwsem: Add missing ACQUIRE to read

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Message-Id: <20190718135206.012378477@infradead.org>
Date:   Thu, 18 Jul 2019 15:49:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Jan Stancek <jstancek@...hat.com>,
        Waiman Long <longman@...hat.com>, dbueso@...e.de,
        mingo@...hat.com, jade.alglave@....com, paulmck@...ux.vnet.ibm.com
Subject: [PATCH 2/4] rwsem: Add missing ACQUIRE to read_slowpath sleep loop

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'.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reported-by: Will Deacon <will@...nel.org>
Acked-by: Will Deacon <will@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/locking/rwsem.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1069,8 +1069,10 @@ rwsem_down_read_slowpath(struct rw_semap
 	/* 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)