From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hangyu Hua Date: Fri, 29 Jul 2022 10:34:39 +0000 Subject: Re: [PATCH] dccp: put dccp_qpolicy_full() and dccp_qpolicy_push() in the same lock Message-Id: List-Id: References: <20220727080609.26532-1-hbh25y@gmail.com> In-Reply-To: <20220727080609.26532-1-hbh25y@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: dccp@vger.kernel.org On 2022/7/29 11:01, Jakub Kicinski wrote: > On Wed, 27 Jul 2022 16:06:09 +0800 Hangyu Hua wrote: >> In the case of sk->dccps_qpolicy = DCCPQ_POLICY_PRIO, dccp_qpolicy_full >> will drop a skb when qpolicy is full. And the lock in dccp_sendmsg is >> released before sock_alloc_send_skb and then relocked after >> sock_alloc_send_skb. The following conditions may lead dccp_qpolicy_push >> to add skb to an already full sk_write_queue: >> >> thread1--->lock >> thread1--->dccp_qpolicy_full: queue is full. drop a skb > > This linie should say "not full"? dccp_qpolicy_full only call dccp_qpolicy_drop when queue is full. You can check out qpolicy_prio_full. qpolicy_prio_full will drop a skb to make suer there is enough space for the next data. So I think it should be "full" here. > >> thread1--->unlock >> thread2--->lock >> thread2--->dccp_qpolicy_full: queue is not full. no need to drop. >> thread2--->unlock >> thread1--->lock >> thread1--->dccp_qpolicy_push: add a skb. queue is full. >> thread1--->unlock >> thread2--->lock >> thread2--->dccp_qpolicy_push: add a skb! >> thread2--->unlock >> >> Fix this by moving dccp_qpolicy_full. >> >> Fixes: 871a2c16c21b ("dccp: Policy-based packet dequeueing infrastructure") > > This code was added in b1308dc015eb0, AFAICT. Please double check. > My fault. I will fix this. >> Signed-off-by: Hangyu Hua >> --- >> net/dccp/proto.c | 10 +++++----- >> 1 file changed, 5 insertions(+), 5 deletions(-) >> >> diff --git a/net/dccp/proto.c b/net/dccp/proto.c >> index eb8e128e43e8..1a0193823c82 100644 >> --- a/net/dccp/proto.c >> +++ b/net/dccp/proto.c >> @@ -736,11 +736,6 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len) >> >> lock_sock(sk); >> >> - if (dccp_qpolicy_full(sk)) { >> - rc = -EAGAIN; >> - goto out_release; >> - } >> - >> timeo = sock_sndtimeo(sk, noblock); >> >> /* >> @@ -773,6 +768,11 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len) >> if (rc != 0) >> goto out_discard; >> >> + if (dccp_qpolicy_full(sk)) { >> + rc = -EAGAIN; >> + goto out_discard; >> + } > > Shouldn't this be earlier, right after relocking? Why copy the data etc. > if we know the queue is full? > You are right. The queue should be checked first after relocking. I will send a v2 later. Thanks, Hangyu. >> dccp_qpolicy_push(sk, skb); >> /* >> * The xmit_timer is set if the TX CCID is rate-based and will expire >