[PATCH 2/2] io_uring: kbuf: add comments for some tricky code

[Hao Xu <hao.xu@linux.dev>]

From: Hao Xu <howeyxu@tencent.com>

Add comments to explain why it is always under uring lock when
incrementing head in __io_kbuf_recycle. And rectify one comemnt about
kbuf consuming in iowq case.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 io_uring/kbuf.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index 9cdbc018fd64..37f06456bf30 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -50,6 +50,13 @@ void __io_kbuf_recycle(struct io_kiocb *req, unsigned issue_flags)
 	if (req->flags & REQ_F_BUFFER_RING) {
 		if (req->buf_list) {
 			if (req->flags & REQ_F_PARTIAL_IO) {
+				/*
+				 * if we reach here, uring_lock has been
+				¦* holden. Because in iowq, we already
+				¦* cleared req->buf_list to NULL when got
+				¦* the buffer from the ring, which means
+				¦* we cannot be here in that case.
+				 */
 				req->buf_list->head++;
 				req->buf_list = NULL;
 			} else {
@@ -128,12 +135,13 @@ static void __user *io_ring_buffer_select(struct io_kiocb *req, size_t *len,
 	if (issue_flags & IO_URING_F_UNLOCKED) {
 		/*
 		 * If we came in unlocked, we have no choice but to consume the
-		 * buffer here. This does mean it'll be pinned until the IO
-		 * completes. But coming in unlocked means we're in io-wq
-		 * context, hence there should be no further retry. For the
-		 * locked case, the caller must ensure to call the commit when
-		 * the transfer completes (or if we get -EAGAIN and must poll
-		 * or retry).
+		 * buffer here otherwise nothing ensures the buffer not being
+		 * used by others. This does mean it'll be pinned until the IO
+		 * completes though coming in unlocked means we're in io-wq
+		 * context and there may be further retries in async hybrid mode.
+		 * For the locked case, the caller must ensure to call the commit
+		 * when the transfer completes (or if we get -EAGAIN and must
+		 * poll or retry).
 		 */
 		req->buf_list = NULL;
 		bl->head++;
-- 
2.25.1