[PATCH] block: ensure the memory order between bi_private and bi_status

[Hou Tao <houtao1@huawei.com>]

When running stress test on null_blk under linux-4.19.y, the following
warning is reported:

  percpu_ref_switch_to_atomic_rcu: percpu ref (css_release) <= 0 (-3) after switching to atomic

The cause is that css_put() is invoked twice on the same bio as shown below:

CPU 1:                         CPU 2:

// IO completion kworker       // IO submit thread
                               __blkdev_direct_IO_simple
                                 submit_bio

bio_endio
  bio_uninit(bio)
    css_put(bi_css)
    bi_css = NULL
                               set_current_state(TASK_UNINTERRUPTIBLE)
  bio->bi_end_io
    blkdev_bio_end_io_simple
      bio->bi_private = NULL
                               // bi_private is NULL
                               READ_ONCE(bio->bi_private)
        wake_up_process
          smp_mb__after_spinlock

                               bio_unint(bio)
                                 // read bi_css as no-NULL
                                 // so call css_put() again
                                 css_put(bi_css)

Because there is no memory barriers between the reading and the writing of
bi_private and bi_css, so reading bi_private as NULL can not guarantee
bi_css will also be NULL on weak-memory model host (e.g, ARM64).

For the latest kernel source, css_put() has been removed from bio_unint(),
but the memory-order problem still exists, because the order between
bio->bi_private and {bi_status|bi_blkg} is also assumed in
__blkdev_direct_IO_simple(). It is reproducible that
__blkdev_direct_IO_simple() may read bi_status as 0 event if
bi_status is set as an errno in req_bio_endio().

In __blkdev_direct_IO(), the memory order between dio->waiter and
dio->bio.bi_status is not guaranteed neither. Until now it is unable to
reproduce it, maybe because dio->waiter and dio->bio.bi_status are
in the same cache-line. But it is better to add guarantee for memory
order.

Fixing it by using smp_load_acquire() & smp_store_release() to guarantee
the order between {bio->bi_private|dio->waiter} and {bi_status|bi_blkg}.

Fixes: 189ce2b9dcc3 ("block: fast-path for small and simple direct I/O requests")
Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 fs/block_dev.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index eb34f5c357cf..a602c6315b0b 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -224,7 +224,11 @@ static void blkdev_bio_end_io_simple(struct bio *bio)
 {
 	struct task_struct *waiter = bio->bi_private;
 
-	WRITE_ONCE(bio->bi_private, NULL);
+	/*
+	 * Paired with smp_load_acquire in __blkdev_direct_IO_simple()
+	 * to ensure the order between bi_private and bi_xxx
+	 */
+	smp_store_release(&bio->bi_private, NULL);
 	blk_wake_io_task(waiter);
 }
 
@@ -283,7 +287,8 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
 	qc = submit_bio(&bio);
 	for (;;) {
 		set_current_state(TASK_UNINTERRUPTIBLE);
-		if (!READ_ONCE(bio.bi_private))
+		/* Refer to comments in blkdev_bio_end_io_simple() */
+		if (!smp_load_acquire(&bio.bi_private))
 			break;
 		if (!(iocb->ki_flags & IOCB_HIPRI) ||
 		    !blk_poll(bdev_get_queue(bdev), qc, true))
@@ -353,7 +358,12 @@ static void blkdev_bio_end_io(struct bio *bio)
 		} else {
 			struct task_struct *waiter = dio->waiter;
 
-			WRITE_ONCE(dio->waiter, NULL);
+			/*
+			 * Paired with smp_load_acquire() in
+			 * __blkdev_direct_IO() to ensure the order between
+			 * dio->waiter and bio->bi_xxx
+			 */
+			smp_store_release(&dio->waiter, NULL);
 			blk_wake_io_task(waiter);
 		}
 	}
@@ -478,7 +488,8 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 
 	for (;;) {
 		set_current_state(TASK_UNINTERRUPTIBLE);
-		if (!READ_ONCE(dio->waiter))
+		/* Refer to comments in blkdev_bio_end_io */
+		if (!smp_load_acquire(&dio->waiter))
 			break;
 
 		if (!(iocb->ki_flags & IOCB_HIPRI) ||
-- 
2.29.2