dlm: fix send buffer backpressure handling

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 <teigland@redhat.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20260929071743.23624-2-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions