rxrpc: peer->mtu_lock is redundant

The peer->mtu_lock is only used to lock around writes to peer->max_data -
and nothing else; further, all such writes take place in the I/O thread and
the lock is only ever write-locked and never read-locked.

In a couple of places, the write_seqcount_begin() is wrapped in
preempt_disable/enable(), but not in all places. This can cause lockdep to
complain:

WARNING: CPU: 0 PID: 1549 at include/linux/seqlock.h:221 rxrpc_input_ack_trailer+0x305/0x430
...
RIP: 0010:rxrpc_input_ack_trailer+0x305/0x430

Fix this by just getting rid of the lock.

Fixes: eeaedc5449d9 ("rxrpc: Implement path-MTU probing using padded PING ACKs (RFC8899)")
Signed-off-by: David Howells <[e-mail]>
cc: Marc Dionne <[e-mail]>
cc: Simon Horman <[e-mail]>
cc: [e-mail]
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/rxrpc/ar-internal.h

···
 	u8			pmtud_jumbo;	/* Max jumbo packets for the MTU */
 	bool			ackr_adv_pmtud;	/* T if the peer advertises path-MTU */
 	unsigned int		ackr_max_data;	/* Maximum data advertised by peer */
-	seqcount_t		mtu_lock;	/* Lockless MTU access management */
 	unsigned int		if_mtu;		/* Local interface MTU (- hdrsize) for this peer */
 	unsigned int		max_data;	/* Maximum packet data capacity for this peer */
 	unsigned short		hdrsize;	/* header size (IP + UDP + RxRPC) */

net/rxrpc/input.c

···
 	if (max_mtu < peer->max_data) {
 		trace_rxrpc_pmtud_reduce(peer, sp->hdr.serial, max_mtu,
 					 rxrpc_pmtud_reduce_ack);
-		write_seqcount_begin(&peer->mtu_lock);
 		peer->max_data = max_mtu;
-		write_seqcount_end(&peer->mtu_lock);
 	}
 
 	max_data = umin(max_mtu, peer->max_data);

net/rxrpc/peer_event.c

···
 			peer->pmtud_bad = max_data + 1;
 
 		trace_rxrpc_pmtud_reduce(peer, 0, max_data, rxrpc_pmtud_reduce_icmp);
-		write_seqcount_begin(&peer->mtu_lock);
 		peer->max_data = max_data;
-		write_seqcount_end(&peer->mtu_lock);
 	}
 }
 
···
 	}
 
 	max_data = umin(max_data, peer->ackr_max_data);
-	if (max_data != peer->max_data) {
-		preempt_disable();
-		write_seqcount_begin(&peer->mtu_lock);
+	if (max_data != peer->max_data)
 		peer->max_data = max_data;
-		write_seqcount_end(&peer->mtu_lock);
-		preempt_enable();
-	}
 
 	jumbo = max_data + sizeof(struct rxrpc_jumbo_header);
 	jumbo /= RXRPC_JUMBO_SUBPKTLEN;

net/rxrpc/peer_object.c

···
 		peer->service_conns = RB_ROOT;
 		seqlock_init(&peer->service_conn_lock);
 		spin_lock_init(&peer->lock);
-		seqcount_init(&peer->mtu_lock);
 		peer->debug_id = atomic_inc_return(&rxrpc_debug_id);
 		peer->recent_srtt_us = UINT_MAX;
 		peer->cong_ssthresh = RXRPC_TX_MAX_WINDOW;