7.3-stable review patch. If anyone has any objections, please let me know. ------------------ From: Eric Dumazet [ Upstream commit e234d7625598b2eaded84fa99f53c8062650828f ] 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@vger.kernel.org Cc: David Teigland Acked-by: Alexander Aring Reviewed-by: Kuniyuki Iwashima Signed-off-by: Eric Dumazet Link: https://patch.msgid.link/20260929071743.23624-2-edumazet@kernel.org Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- 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 2aff1c7c17de4..abe9ae4c643fa 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. -- 2.53.0