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
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")
Signed-off-by: Hangyu Hua <[email protected]>
---
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;
+ }
+
dccp_qpolicy_push(sk, skb);
/*
* The xmit_timer is set if the TX CCID is rate-based and will expire
--
2.25.1
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"?
> 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.
> Signed-off-by: Hangyu Hua <[email protected]>
> ---
> 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?
> dccp_qpolicy_push(sk, skb);
> /*
> * The xmit_timer is set if the TX CCID is rate-based and will expire
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 <[email protected]>
>> ---
>> 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
>
On Fri, 29 Jul 2022 18:34:39 +0800 Hangyu Hua wrote:
> >> 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.
Oh, I see what you're saying. That's unnecessarily complicated,
I reckon. The "simple" policy suffers from the same problem and
is easier to understand. Anyway, you already sent v2 and it doesn't
matter enough to warrant v3, so fine.