On Tue, Jan 08, 2019 at 09:56:32AM -0700, Jens Axboe wrote:
> For the upcoming async polled IO, we can't sleep allocating requests.
> If we do, then we introduce a deadlock where the submitter already
> has async polled IO in-flight, but can't wait for them to complete
> since polled requests must be active found and reaped.
>
> Utilize the helper in the blockdev DIRECT_IO code.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> ---
> fs/block_dev.c | 4 ++--
> include/linux/bio.h | 14 ++++++++++++++
> 2 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 5415579f3e14..2ebd2a0d7789 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -233,7 +233,7 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
> task_io_account_write(ret);
> }
> if (iocb->ki_flags & IOCB_HIPRI)
> - bio.bi_opf |= REQ_HIPRI;
> + bio_set_polled(&bio, iocb);
>
> qc = submit_bio(&bio);
> for (;;) {
> @@ -401,7 +401,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
> nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
> if (!nr_pages) {
> if (iocb->ki_flags & IOCB_HIPRI)
> - bio->bi_opf |= REQ_HIPRI;
> + bio_set_polled(bio, iocb);
>
> qc = submit_bio(bio);
> WRITE_ONCE(iocb->ki_cookie, qc);
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 7380b094dcca..f6f0a2b3cbc8 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -823,5 +823,19 @@ static inline int bio_integrity_add_page(struct bio *bio, struct page *page,
>
> #endif /* CONFIG_BLK_DEV_INTEGRITY */
>
> +/*
> + * Mark a bio as polled. Note that for async polled IO, the caller must
> + * expect -EWOULDBLOCK if we cannot allocate a request (or other resources).
> + * We cannot block waiting for requests on polled IO, as those completions
> + * must be found by the caller. This is different than IRQ driven IO, where
> + * it's safe to wait for IO to complete.
> + */
> +static inline void bio_set_polled(struct bio *bio, struct kiocb *kiocb)
> +{
> + bio->bi_opf |= REQ_HIPRI;
> + if (!is_sync_kiocb(kiocb))
> + bio->bi_opf |= REQ_NOWAIT;
> +}
> +
REQ_NOWAIT doesn't cover allocating split bio, is that a issue?
BTW, could you explain a bit about the deadlock in case of sleep from
request allocation?
Thanks,
Ming
