ping ? On 7/1/2021 7:35 PM, 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") > 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) ||