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For n:1 'datagram connections' (eg /dev/log), the unix_dgram_sendmsg routine implements a 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 PF_UNIX non-stream sockets connected to server sockets dealing with (potentially) multiple clients if 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_recvmsg 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: Rainer Weikusat <rweikusat@mssgmbh.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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| af_unix.c | ||
| garbage.c | ||
| Kconfig | ||
| Makefile | ||
| sysctl_net_unix.c | ||