[PATCH 1/2] io_uring/rw: don't allow multishot reads without NOWAIT support

[Jens Axboe <axboe@kernel.dk>]

Supporting multishot reads requires support for NOWAIT, as the
alternative would be always having io-wq execute the work item whenever
the poll readiness triggered. Any fast file type will have NOWAIT
support (eg it understands both O_NONBLOCK and IOCB_NOWAIT). If the
given file type does not, then simply resort to single shot execution.

Cc: stable@vger.kernel.org
Fixes: fc68fcda04910 ("io_uring/rw: add support for IORING_OP_READ_MULTISHOT")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/rw.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/io_uring/rw.c b/io_uring/rw.c
index 0585ebcc9773..c8d48287439e 100644
--- a/io_uring/rw.c
+++ b/io_uring/rw.c
@@ -936,6 +936,13 @@ int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
 
 	ret = __io_read(req, issue_flags);
 
+	/*
+	 * If the file doesn't support proper NOWAIT, then disable multishot
+	 * and stay in single shot mode.
+	 */
+	if (!io_file_supports_nowait(req))
+		req->flags &= ~REQ_F_APOLL_MULTISHOT;
+
 	/*
 	 * If we get -EAGAIN, recycle our buffer and just let normal poll
 	 * handling arm it.
@@ -955,7 +962,7 @@ int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
 	/*
 	 * Any successful return value will keep the multishot read armed.
 	 */
-	if (ret > 0) {
+	if (ret > 0 && req->flags & REQ_F_APOLL_MULTISHOT) {
 		/*
 		 * Put our buffer and post a CQE. If we fail to post a CQE, then
 		 * jump to the termination path. This request is then done.
-- 
2.43.0