[PATCH] dlm: fix send buffer backpressure handling

[Eric Dumazet]

lowcomms.c tests and clears SOCKWQ_ASYNC_NOSPACE in con->sock->flags,
but this bit has not been stored there for ten years.

Commit 9cd3e072b0be ("net: rename SOCK_ASYNC_NOSPACE and
SOCK_ASYNC_WAITDATA") mechanically renamed the two dlm users, then
commit ceb5d58b2170 ("net: fix sock_wake_async() rcu protection") moved
the bit from socket->flags to the RCU protected socket_wq->flags, where
it is reachable only through sk_set_bit() and sk_clear_bit(), and is
only maintained for sockets having SOCK_FASYNC set.  dlm uses kernel
sockets, which never have SOCK_FASYNC set, and never sets the bit
itself, so the test in send_to_sock() has been false ever since.

The consequence is that when sock_sendmsg() returns -EAGAIN because the
socket send buffer is full, dlm no longer sets CF_APP_LIMITED, does not
increment sk_write_pending, and does not return DLM_IO_END to wait for
lowcomms_write_space().  It returns DLM_IO_RESCHED instead, and
process_send_sockets() immediately requeues the send work.  A connection
to a peer that is slow to drain thus keeps cycling through
sock_sendmsg() and -EAGAIN, burning CPU, instead of sleeping until TCP
reports that space is available again.

Test SOCK_NOSPACE instead.  This is the bit that lives in socket->flags,
that TCP sets whenever sendmsg() returns -EAGAIN for lack of send buffer
space (tcp_sendmsg_locked() and sk_stream_wait_memory()), and that
lowcomms_write_space() already clears.  This restores the semantics dlm
had before the bit moved.

Also remove the clear_bit() of SOCKWQ_ASYNC_NOSPACE from
lowcomms_write_space(), for the same reason.

SCTP connections are deliberately left as they are.  SCTP does not set
SOCK_NOSPACE, and it never calls sk->sk_write_space(): sctp_wfree() ends
up in sctp_wake_up_waiters(), which calls sctp_write_space() directly.
lowcomms_write_space() is thus never invoked for an SCTP connection, and
the test added here stays false, so send_to_sock() keeps returning
DLM_IO_RESCHED as it does today.  This is the only safe behavior, as
returning DLM_IO_END would wait for a callback that never comes.

Fixes: ceb5d58b2170 ("net: fix sock_wake_async() rcu protection")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
---
 fs/dlm/lowcomms.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/dlm/lowcomms.c b/fs/dlm/lowcomms.c
index 2aff1c7c17de..abe9ae4c643f 100644
--- a/fs/dlm/lowcomms.c
+++ b/fs/dlm/lowcomms.c
@@ -522,10 +522,8 @@ static void lowcomms_write_space(struct sock *sk)
 	clear_bit(SOCK_NOSPACE, &con->sock->flags);
 
 	spin_lock_bh(&con->writequeue_lock);
-	if (test_and_clear_bit(CF_APP_LIMITED, &con->flags)) {
+	if (test_and_clear_bit(CF_APP_LIMITED, &con->flags))
 		con->sock->sk->sk_write_pending--;
-		clear_bit(SOCKWQ_ASYNC_NOSPACE, &con->sock->flags);
-	}
 
 	lowcomms_queue_swork(con);
 	spin_unlock_bh(&con->writequeue_lock);
@@ -1391,7 +1389,7 @@ static int send_to_sock(struct connection *con)
 	if (ret == -EAGAIN || ret == 0) {
 		lock_sock(con->sock->sk);
 		spin_lock_bh(&con->writequeue_lock);
-		if (test_bit(SOCKWQ_ASYNC_NOSPACE, &con->sock->flags) &&
+		if (test_bit(SOCK_NOSPACE, &con->sock->flags) &&
 		    !test_and_set_bit(CF_APP_LIMITED, &con->flags)) {
 			/* Notify TCP that we're limited by the
 			 * application window size.
-- 
2.55.0.1007.g17ff1f9808-goog