Messages in this thread | | | Date | Tue, 17 Jun 2008 21:56:30 -0700 (PDT) | Subject | Re: [PATCH 2.6.25.7] af_unix: fix 'poll for write'/ connected DGRAM sockets | From | David Miller <> |
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From: Rainer Weikusat <rweikusat@mssgmbh.com> Date: Tue, 17 Jun 2008 20:47:02 +0200
> The unix_dgram_sendmsg routine implements a (somewhat crude) > form of receiver-imposed flow control by comparing the length of the > receive queue of the 'peer socket' with the max_ack_backlog value > stored in the corresponding sock structure, either blocking > the thread which caused the send-routine to be called or returning > EAGAIN. This routine is used by both SOCK_DGRAM and SOCK_SEQPACKET > sockets. The poll-implementation for these socket types is > datagram_poll from core/datagram.c. A socket is deemed to be writeable > by this routine when the memory presently consumed by datagrams > owned by it is less than the configured socket send buffer size. This > is always wrong for connected PF_UNIX non-stream sockets when the > abovementioned receive queue is currently considered to be full. > 'poll' will then return, indicating that the socket is writeable, but > a subsequent write result in EAGAIN, effectively causing an > (usual) application to 'poll for writeability by repeated send request > with O_NONBLOCK set' until it has consumed its time quantum. > > The change below uses a suitably modified variant of the datagram_poll > routines for both type of PF_UNIX sockets, which tests if the > recv-queue of the peer a socket is connected to is presently > considered to be 'full' as part of the 'is this socket > writeable'-checking code. The socket being polled is additionally > put onto the peer_wait wait queue associated with its peer, because the > unix_dgram_sendmsg routine does a wake up on this queue after a > datagram was received and the 'other wakeup call' is done implicitly > as part of skb destruction, meaning, a process blocked in poll > because of a full peer receive queue could otherwise sleep forever > if no datagram owned by its socket was already sitting on this queue. > Among this change is a small (inline) helper routine named > 'unix_recvq_full', which consolidates the actual testing code (in three > different places) into a single location. > > Signed-off-by: <rweikusat@mssgmbh.com>
Thank you for fixing this bug.
I'm going to review the logic in the new poll routing a little bit more, then apply it to net-2.6 unless I find some problems.
Thanks again.
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