netdev - Re: [PATCH net-next] RDS: deliver zerocopy completion notification with data as an optimization

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Message-ID: <CAF=yD-+oihB6Opusn1hBVsEvk654BEg2LN6vXUtwS4q+2gkivQ@mail.gmail.com>
Date:   Wed, 21 Feb 2018 18:45:43 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>
Subject: Re: [PATCH net-next] RDS: deliver zerocopy completion notification
 with data as an optimization

>> Okay. If callers must already handle 0 as a valid return code, then
>> it is fine to add another case that does this.
>>
>> The extra branch in the hot path is still rather unfortunately. Could
>> this be integrated in the existing if (nonblock) branch below?
>
> that's where I first started. it got even hairier because the
> callers expect a retval of 0 (-EAGAIN threw rds-stress into an
> infinite loop of continulally trying to recv) and the end result
> was just confusing code with the same number of branches..

I do mean returning 0 instead of -EAGAIN if control data is ready.
Something like

@@ -611,7 +611,8 @@ int rds_recvmsg(struct socket *sock, struct msghdr
*msg, size_t size,

                if (!rds_next_incoming(rs, &inc)) {
                        if (nonblock) {
-                               ret = -EAGAIN;
+                               ncookies = rds_recvmsg_zcookie(rs, msg);
+                               ret = ncookies ? 0 : -EAGAIN;
                                break;
                        }

By the way, the put_cmsg is unconditional even if the caller did
not supply msg_control. So it is basically no longer safe to ever
call read, recv or recvfrom on a socket if zerocopy notifications
are outstanding.

It is possible to check msg_controllen before even deciding whether
to try to dequeue notifications (and take the lock). I see that this is
not common. But RDS of all cases seems to do this, in
rds_notify_queue_get:

                max_messages = msghdr->msg_controllen /
CMSG_SPACE(sizeof(cmsg));