[PATCH 03/15] block: add bio_set_polled() helper

From: Jens Axboe <axboe@kernel.dk>
To: linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
	linux-block@vger.kernel.org, linux-arch@vger.kernel.org
Cc: hch@lst.de, jmoyer@redhat.com, avi@scylladb.com,
	Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 03/15] block: add bio_set_polled() helper
Date: Wed, 16 Jan 2019 10:49:51 -0700
Message-ID: <20190116175003.17880-4-axboe@kernel.dk> (raw)
In-Reply-To: <20190116175003.17880-1-axboe@kernel.dk>

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@kernel.dk>
---
 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;
+}
+
 #endif /* CONFIG_BLOCK */
 #endif /* __LINUX_BIO_H */
-- 
2.17.1

