From: Yu Kuai <yukuai3@huawei.com> hulk inclusion category: bugfix bugzilla: 167067 https://gitee.com/openeuler/kernel/issues/I4DDEL -------------------------------- 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_wmb() and spm_rmb() 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: Yu Kuai <yukuai3@huawei.com> Reviewed-by: Hou Tao <houtao1@huawei.com> Signed-off-by: Chen Jun <chenjun102@huawei.com> --- fs/block_dev.c | 24 ++++++++++++++++++++++-- 1 file changed, 22 insertions(+), 2 deletions(-) diff --git a/fs/block_dev.c b/fs/block_dev.c index 7bc660054d21..d2881fd351a3 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -229,6 +229,11 @@ static void blkdev_bio_end_io_simple(struct bio *bio) { struct task_struct *waiter = bio->bi_private; + /* + * Paired with smp_rmb() in __blkdev_direct_IO_simple() to ensure + * the order between bi_private and bi_xxx. + */ + smp_wmb(); WRITE_ONCE(bio->bi_private, NULL); blk_wake_io_task(waiter); } @@ -288,8 +293,15 @@ __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)) + if (!READ_ONCE(bio.bi_private)) { + /* + * Paired with smp_wmb() in + * blkdev_bio_end_io_simple(). + */ + smp_rmb(); break; + } + if (!(iocb->ki_flags & IOCB_HIPRI) || !blk_poll(bdev_get_queue(bdev), qc, true)) blk_io_schedule(); @@ -358,6 +370,11 @@ static void blkdev_bio_end_io(struct bio *bio) } else { struct task_struct *waiter = dio->waiter; + /* + * Paired with smp_rmb() in __blkdev_direct_IO() to + * ensure the order between dio->waiter and bio->bi_xxx. + */ + smp_wmb(); WRITE_ONCE(dio->waiter, NULL); blk_wake_io_task(waiter); } @@ -483,8 +500,11 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages) for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); - if (!READ_ONCE(dio->waiter)) + if (!READ_ONCE(dio->waiter)) { + /* Paired with smp_wmb() in blkdev_bio_end_io(). */ + smp_rmb(); break; + } if (!(iocb->ki_flags & IOCB_HIPRI) || !blk_poll(bdev_get_queue(bdev), qc, true)) -- 2.20.1