block: don't make REQ_POLLED imply REQ_NOWAIT

Normally these two flags do go together, as the issuer of polled IO
generally cannot wait for resources that will get freed as part of IO
completion. This is because that very task is the one that will complete
the request and free those resources, hence that would introduce a
deadlock.

But it is possible to have someone else issue the polled IO, eg via
io_uring if the request is punted to io-wq. For that case, it's fine to
have the task block on IO submission, as it is not the same task that
will be completing the IO.

It's completely up to the caller to ask for both polled and nowait IO
separately! If we don't allow polled IO where IOCB_NOWAIT isn't set in
the kiocb, then we can run into repeated -EAGAIN submissions and not
make any progress.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/fops.c b/block/fops.c
index a286bf3325c5d84bf8d0e388f560f584ec57283c..838ffada53413f4fd40ff154c24b1d7af593a48a 100644 (file)
--- a/block/fops.c
+++ b/block/fops.c
@@ -358,13 +358,14 @@ static ssize_t __blkdev_direct_IO_async(struct kiocb *iocb,
                task_io_account_write(bio->bi_iter.bi_size);
        }
 
+       if (iocb->ki_flags & IOCB_NOWAIT)
+               bio->bi_opf |= REQ_NOWAIT;
+
        if (iocb->ki_flags & IOCB_HIPRI) {
-               bio->bi_opf |= REQ_POLLED | REQ_NOWAIT;
+               bio->bi_opf |= REQ_POLLED;
                submit_bio(bio);
                WRITE_ONCE(iocb->private, bio);
        } else {
-               if (iocb->ki_flags & IOCB_NOWAIT)
-                       bio->bi_opf |= REQ_NOWAIT;
                submit_bio(bio);
        }
        return -EIOCBQUEUED;

diff --git a/include/linux/bio.h b/include/linux/bio.h
index c4f5b5228105fe2a7f8c322da3a511cf44e5697c..11984ed29cb8f372ed5cf58511cbf80232965d0c 100644 (file)
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -791,7 +791,7 @@ static inline int bio_integrity_add_page(struct bio *bio, struct page *page,
 static inline void bio_set_polled(struct bio *bio, struct kiocb *kiocb)
 {
        bio->bi_opf |= REQ_POLLED;
-       if (!is_sync_kiocb(kiocb))
+       if (kiocb->ki_flags & IOCB_NOWAIT)
                bio->bi_opf |= REQ_NOWAIT;
 }