[PATCH v2 03/20] smb: server: make use of smbdirect_socket.recv_io.credits.available

[Stefan Metzmacher]

The logic off managing recv credits by counting posted recv_io and
granted credits is racy.

That's because the peer might already consumed a credit,
but between receiving the incoming recv at the hardware
and processing the completion in the 'recv_done' functions
we likely have a window where we grant credits, which
don't really exist.

So we better have a decicated counter for the
available credits, which will be incremented
when we posted new recv buffers and drained when
we grant the credits to the peer.

This fixes regression Namjae reported with
the 6.18 release.

Fixes: 89b021a72663 ("smb: server: manage recv credits by counting posted recv_io and granted credits")
Cc: <stable at vger.kernel.org> # 6.18.x
Cc: Namjae Jeon <linkinjeon at kernel.org>
Cc: Steve French <smfrench at gmail.com>
Cc: Tom Talpey <tom at talpey.com>
Cc: linux-cifs at vger.kernel.org
Cc: samba-technical at lists.samba.org
Signed-off-by: Stefan Metzmacher <metze at samba.org>
Acked-by: Namjae Jeon <linkinjeon at kernel.org>
---
 fs/smb/server/transport_rdma.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/fs/smb/server/transport_rdma.c b/fs/smb/server/transport_rdma.c
index 05f008ea51cd..c7abd621bd11 100644
--- a/fs/smb/server/transport_rdma.c
+++ b/fs/smb/server/transport_rdma.c
@@ -1028,6 +1028,8 @@ static void smb_direct_post_recv_credits(struct work_struct *work)
 		}
 	}
 
+	atomic_add(credits, &sc->recv_io.credits.available);
+
 	if (credits)
 		queue_work(sc->workqueue, &sc->idle.immediate_work);
 }
@@ -1074,19 +1076,37 @@ static void send_done(struct ib_cq *cq, struct ib_wc *wc)
 
 static int manage_credits_prior_sending(struct smbdirect_socket *sc)
 {
+	int missing;
+	int available;
 	int new_credits;
 
 	if (atomic_read(&sc->recv_io.credits.count) >= sc->recv_io.credits.target)
 		return 0;
 
-	new_credits = atomic_read(&sc->recv_io.posted.count);
-	if (new_credits == 0)
+	missing = (int)sc->recv_io.credits.target - atomic_read(&sc->recv_io.credits.count);
+	available = atomic_xchg(&sc->recv_io.credits.available, 0);
+	new_credits = (u16)min3(U16_MAX, missing, available);
+	if (new_credits <= 0) {
+		/*
+		 * If credits are available, but not granted
+		 * we need to re-add them again.
+		 */
+		if (available)
+			atomic_add(available, &sc->recv_io.credits.available);
 		return 0;
+	}
 
-	new_credits -= atomic_read(&sc->recv_io.credits.count);
-	if (new_credits <= 0)
-		return 0;
+	if (new_credits < available) {
+		/*
+		 * Readd the remaining available again.
+		 */
+		available -= new_credits;
+		atomic_add(available, &sc->recv_io.credits.available);
+	}
 
+	/*
+	 * Remember we granted the credits
+	 */
 	atomic_add(new_credits, &sc->recv_io.credits.count);
 	return new_credits;
 }
-- 
2.43.0