Messages in this thread Patch in this message | | | Date | Thu, 16 Nov 2023 15:19:51 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather than read_seqbegin_or_lock() |
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On 11/16, David Howells wrote: > > Oleg Nesterov <oleg@redhat.com> wrote: > > > So do you agree that > > > > - the usage of read_seqbegin_or_lock/need_seqretry in > > this code makes no sense because read_seqlock_excl() > > is not possible > > Not exactly. I think it should take a lock on the second pass.
OK, then how about the patch below?
Again, I'd prefer to change the semantics/prototype of need_seqretry() to enforce the locking on the 2nd pass "automatically", but a) this needs more discussion and b) I can't do this before I update the users which use read_seqbegin_or_lock/need_seqretry incorrectly. So lets discuss this later.
Oleg.
--- a/net/rxrpc/conn_service.c +++ b/net/rxrpc/conn_service.c @@ -25,7 +25,7 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer, struct rxrpc_conn_proto k; struct rxrpc_skb_priv *sp = rxrpc_skb(skb); struct rb_node *p; - unsigned int seq = 0; + unsigned int seq = 1; k.epoch = sp->hdr.epoch; k.cid = sp->hdr.cid & RXRPC_CIDMASK; @@ -35,6 +35,7 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer, * under just the RCU read lock, so we have to check for * changes. */ + seq++; /* 2 on the 1st/lockless path, otherwise odd */ read_seqbegin_or_lock(&peer->service_conn_lock, &seq); p = rcu_dereference_raw(peer->service_conns.rb_node);
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