[PATCH net v5 6/7] net/rds: acquire the fastpath locks in rds_conn_shutdown()

[Allison Henderson <achender@kernel.org>]

From: Håkon Bugge <haakon.bugge@oracle.com>

rds_conn_shutdown() quiesces the transmit and receive-refill paths by
waiting for RDS_IN_XMIT and RDS_RECV_REFILL to be sampled clear, and
then runs the transport shutdown and rds_conn_path_reset().  Sampling
the bits clear is not the same as owning them: the moment after the
wait_event() returns, rds_send_xmit() can re-acquire RDS_IN_XMIT (or
rds_ib_recv_refill() can re-acquire RDS_RECV_REFILL) and run
concurrently with the teardown.

The sender does recheck the connection state after taking the lock,
but that recheck is a classic store-buffering pattern: teardown writes
the state and reads the bit while the sender writes the bit and reads
the state.  acquire_in_xmit() is only an acquire operation, so on
weakly ordered architectures both sides can miss each other's write,
and the transmit path then runs while the transport zeroes its rings
(e.g. rds_ib_ring_init()) and rds_send_path_reset() rewrites the
transmit state under it.

Oracle UEK fixed the same class of crashes - a 14-year tail of
BUG_ON()s in rds_ib_sub_signaled(), unexpected op-codes and NULL
dereferences in rds_ib_send_cqe_handler() during failover testing -
by making the teardown path *acquire* the fastpath bit locks instead
of testing them ("rds: Make sure transmit path and connection
tear-down does not run concurrently").  Ownership of a single word is
decided by RMW atomicity, so no cross-variable ordering is needed.

Do the same here: take both locks before calling the transport
shutdown, hold them across rds_conn_path_reset(), and release them
explicitly with a wake-up afterwards.  Both are released with
clear_bit_unlock(), so that the ring re-initialization done by the
transport shutdown and the transmit state rewritten by
rds_send_path_reset() are ordered before either bit is seen clear by
the next acquire_in_xmit() or acquire_refill().

The fastpath users of these bits - rds_send_xmit() and
rds_ib_recv_refill() - are trylock style and back off while teardown
owns the locks, so no new lock dependency is introduced for them.
rds_tcp_reset_callbacks() is different: since the previous patch it
acquires RDS_IN_XMIT as well, and it blocks doing so, so its wait now
spans the teardown instead of at most one send batch.  That waiter
runs from rds_tcp_accept_one() on the single-threaded krdsd workqueue
and holds rds_tcp_accept_lock and t_conn_path_lock while it waits, so
a duelling SYN accepted while its path is being torn down parks
accept processing for the duration of the teardown - for TCP bounded
by the (up to 5 s) drain loop in rds_tcp_conn_path_shutdown().  An IB
path's drain in rds_ib_conn_path_shutdown() has no round cap, but no
blocking waiter either: rds_tcp_reset_callbacks() is the only blocking
acquirer of these bits and waits only on its own TCP path, and the
fastpaths are trylock-and-back-off on both transports, so a long IB
drain lengthens only that path's own quiesce.  The
window is narrow: the accept-side state check has to pass before the
teardown moves the path to RDS_CONN_DISCONNECTING.

Because krdsd is a single global workqueue, everything else queued
there - accept processing for other connections and network
namespaces, and the flush_workqueue(rds_wq) in rds_tcp_listen_stop()
during namespace teardown - waits behind the parked accept worker for
that time.  It cannot deadlock, although the waits do point at each
other: the teardown blocks until the bit's holder releases it, and
the holder may be that krdsd accept worker.  The holder finishes
without needing anything the teardown owns: the sync cancels
rds_tcp_reset_callbacks() issues target cp_send_w and cp_recv_w on
the path's ordered cp_wq, whose only execution slot is occupied by
the blocked cp_down_w itself, so they are pending at most and cancel
without flushing - a reliance on cp_wq being ordered that is now
noted next to those cancels (on the allocation-failure fallback where
a path shares rds_wq, the work items simply serialize).
Nor is the blocking wait itself new: rds_tcp_reset_callbacks() has
waited on RDS_IN_XMIT from the krdsd work item since
commit 335b48d980f6 ("RDS: TCP: Add/use rds_tcp_reset_callbacks to
reset tcp socket safely"); this patch stretches its worst case from
a sender's batch to the teardown's drain.  The alternative to parking
is the accept path racing the teardown, which is what these patches
close; making the teardown itself non-blocking is a separate item.

One observable side effect: the SENDING flag reported by rds-info has
always mirrored RDS_IN_XMIT, so it now also covers the window where
teardown owns the bit.

The comments that describe the old sample-based handshake or name
rds_send_xmit() as the only other holder of these bits - in
rds_send_xmit(), above rds_conn_path_reset(), in rds_ib_recv_refill()
and in rds_tcp_reset_callbacks() - are updated to match.

For anyone backporting this patch standalone: it depends on
"net/rds: clear cp_flags bits individually in rds_conn_path_reset()"
and "net/rds: acquire RDS_IN_XMIT in rds_tcp_reset_callbacks()"
earlier in this series.  Without the former, the blanket cp_flags
clear in rds_conn_path_reset() would drop both held bits in the middle
of the teardown; without the latter, rds_tcp_reset_callbacks() would
still sample t_sock without owning RDS_IN_XMIT.  "net/rds: use
clear_bit_unlock() in release_refill()" is needed for the refill
side's release to pair with the acquire added here, and the follow-up
"net/rds: don't let rds_conn_shutdown() consume a concurrent drop"
completes the teardown-state handling for the waiter this patch
parks; a backport should carry all four.

Fixes: 0f4b1c7e89e6 ("rds: fix rds_send_xmit() serialization")
Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
[achender: reimplement for net-next shutdown path: acquire the existing
 RDS_IN_XMIT/RDS_RECV_REFILL bit locks in rds_conn_shutdown() and release
 after teardown; update comments and commit message]
Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Allison Henderson <achender@kernel.org>
---
    v5: changelog corrections (backport prerequisites, krdsd wait
    reasoning, TCP scope of the drain bound); the send.c ordering
    comment and the sync-cancel comment in rds_tcp_reset_callbacks()
    updated to match.  No functional change.
 net/rds/connection.c | 40 ++++++++++++++++++++++++++++++++--------
 net/rds/ib_recv.c    |  4 +++-
 net/rds/send.c       |  7 +++++--
 net/rds/tcp.c        | 19 +++++++++++++++----
 4 files changed, 55 insertions(+), 15 deletions(-)

diff --git a/net/rds/connection.c b/net/rds/connection.c
index 46ac72088f84..fbbac55a0e81 100644
--- a/net/rds/connection.c
+++ b/net/rds/connection.c
@@ -106,10 +106,12 @@ static struct rds_connection *rds_conn_lookup(struct net *net,
 }
 
 /*
- * This is called by transports as they're bringing down a connection.
- * It clears partial message state so that the transport can start sending
- * and receiving over this connection again in the future.  It is up to
- * the transport to have serialized this call with its send and recv.
+ * This is called by rds_conn_shutdown() once the transport has brought
+ * a path down.  It clears partial message state so that the transport
+ * can start sending and receiving over this path again in the future.
+ * The caller owns RDS_IN_XMIT and RDS_RECV_REFILL across this call,
+ * which is what serializes it against the send and receive-refill
+ * paths.
  */
 static void rds_conn_path_reset(struct rds_conn_path *cp)
 {
@@ -124,8 +126,9 @@ static void rds_conn_path_reset(struct rds_conn_path *cp)
 	/* Clear the bits the reset is responsible for individually: a
 	 * blanket cp_flags = 0 is a plain store that can clobber a
 	 * concurrent atomic read-modify-write on the same word.
-	 * RDS_IN_XMIT and RDS_RECV_REFILL belong to the caller,
-	 * rds_conn_shutdown(), and are left alone here.
+	 * RDS_IN_XMIT and RDS_RECV_REFILL are held as locks by the
+	 * caller, rds_conn_shutdown(), which releases them once the
+	 * teardown is complete.
 	 */
 	clear_bit(RDS_LL_SEND_FULL, &cp->cp_flags);
 	clear_bit(RDS_RECONNECT_PENDING, &cp->cp_flags);
@@ -414,14 +417,35 @@ void rds_conn_shutdown(struct rds_conn_path *cp)
 		}
 		mutex_unlock(&cp->cp_cm_lock);
 
+		/* Quiesce the transmit and receive-refill paths by
+		 * acquiring their bit locks, not merely waiting for
+		 * them to be released: with a plain wait, either path
+		 * can re-take its lock the instant after we sample it
+		 * clear and then run concurrently with the transport
+		 * shutdown and the path reset below.  Holding both
+		 * locks across the teardown makes that structurally
+		 * impossible.
+		 */
 		wait_event(cp->cp_waitq,
-			   !test_bit(RDS_IN_XMIT, &cp->cp_flags));
+			   !test_and_set_bit_lock(RDS_IN_XMIT, &cp->cp_flags));
 		wait_event(cp->cp_waitq,
-			   !test_bit(RDS_RECV_REFILL, &cp->cp_flags));
+			   !test_and_set_bit(RDS_RECV_REFILL, &cp->cp_flags));
 
 		conn->c_trans->conn_path_shutdown(cp);
 		rds_conn_path_reset(cp);
 
+		/* Release the two locks and wake any waiter (e.g.
+		 * rds_tcp_reset_callbacks()) that blocked on them while
+		 * we held them.  The unlock orders the transport's ring
+		 * re-initialization and the path reset above before
+		 * either bit is seen clear.  rds_conn_path_reset() leaves
+		 * both bits alone: ownership ends here, not inside the
+		 * reset.
+		 */
+		clear_bit_unlock(RDS_IN_XMIT, &cp->cp_flags);
+		clear_bit_unlock(RDS_RECV_REFILL, &cp->cp_flags);
+		wake_up_all(&cp->cp_waitq);
+
 		if (!rds_conn_path_transition(cp, RDS_CONN_DISCONNECTING,
 					      RDS_CONN_DOWN) &&
 		    !rds_conn_path_transition(cp, RDS_CONN_ERROR,
diff --git a/net/rds/ib_recv.c b/net/rds/ib_recv.c
index a6983861eec7..bd6cb3ffaa57 100644
--- a/net/rds/ib_recv.c
+++ b/net/rds/ib_recv.c
@@ -391,7 +391,9 @@ void rds_ib_recv_refill(struct rds_connection *conn, int prefill, gfp_t gfp)
 
 	/* the goal here is to just make sure that someone, somewhere
 	 * is posting buffers.  If we can't get the refill lock,
-	 * let them do their thing
+	 * let them do their thing.  The holder may also be
+	 * rds_conn_shutdown() tearing the path down, in which case
+	 * there is nothing to post.
 	 */
 	if (!acquire_refill(conn))
 		return;
diff --git a/net/rds/send.c b/net/rds/send.c
index 8aad185e4b1a..1afa981e5c06 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -244,8 +244,11 @@ int rds_send_xmit(struct rds_conn_path *cp)
 	WRITE_ONCE(cp->cp_send_gen, send_gen);
 
 	/*
-	 * rds_conn_shutdown() sets the conn state and then tests RDS_IN_XMIT,
-	 * we do the opposite to avoid races.
+	 * rds_conn_shutdown() sets the conn state and then acquires
+	 * RDS_IN_XMIT; we take the lock first and then check the state.
+	 * Ownership is decided by the atomic RMW on the cp_flags word:
+	 * if the teardown won the bit we back off here, and if we won
+	 * it the teardown waits until we release it.
 	 */
 	if (!rds_conn_path_up(cp)) {
 		release_in_xmit(cp);
diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index f4c83e368390..69c6d3145b5a 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c
@@ -144,8 +144,10 @@ void rds_tcp_reset_callbacks(struct socket *sock,
 	 * so we must quiesce any send threads before resetting
 	 * cp_transport_data.  Setting cp_state to something other
 	 * than RDS_CONN_UP stops new senders, and owning RDS_IN_XMIT
-	 * excludes any thread already inside rds_send_xmit() for the
-	 * whole socket swap and the rds_send_path_reset() below.
+	 * excludes any thread already inside rds_send_xmit() - or a
+	 * teardown in rds_conn_shutdown(), which holds the same lock
+	 * for the duration of the transport shutdown - for the whole
+	 * socket swap and the rds_send_path_reset() below.
 	 *
 	 * An incoming syn-ack at this point would end up marking the
 	 * conn as RDS_CONN_UP, and would again permit rds_send_xmit()
@@ -178,13 +180,22 @@ void rds_tcp_reset_callbacks(struct socket *sock,
 	/* Read t_sock only while owning RDS_IN_XMIT, never before the
 	 * wait: the teardown in rds_conn_shutdown() releases the old
 	 * socket and clears t_sock, so a pointer sampled earlier can
-	 * be stale by the time we wake up.
+	 * be stale by the time we wake up.  The teardown holds the
+	 * same lock while it does so, so what we read here cannot
+	 * change under us until we release it.
 	 */
 	osock = tc->t_sock;
 	if (!osock)
 		goto newsock;
 
-	/* reset receive side state for rds_tcp_data_recv() for osock  */
+	/* reset receive side state for rds_tcp_data_recv() for osock.
+	 *
+	 * The sync cancels while owning RDS_IN_XMIT rely on cp_wq
+	 * being ordered: a teardown blocked on the bit occupies
+	 * cp_wq's only execution slot, so cp_send_w and cp_recv_w are
+	 * pending at most and the cancels never flush.  Nothing here
+	 * may flush or wait on cp_wq itself.
+	 */
 	cancel_delayed_work_sync(&cp->cp_send_w);
 	cancel_delayed_work_sync(&cp->cp_recv_w);
 	lock_sock(osock->sk);
-- 
2.25.1