On Thu, Jul 01, 2021 at 07:35:37PM +0800, Hou Tao wrote:
> 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")
This obviously does not look broken, but smp_load_acquire /
smp_store_release is way beyond my paygrade. Adding some CCs.
> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> ---
> 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
---end quoted text---
