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From: Rainer Weikusat on 16 Jun 2008 12:34
As I just had the mispleasure to discover: Assume a connected PF_UNIX
datagram socket on Linux (checked w/ 2.6, applies probably to earlier
versions, too). 'poll' will return, indicating that the socket is
writeable when the amount of memory consumed by skbs owned by it is
less than the size of the write buffer of this socket. But there is
another parameter, called 'max_ack_backlog', associated with the
destination socket and sending a datagram to such a socket when
max_ack_backlog datagrams are already queued for it will either block
or fail with EAGAIN, regardless of the result of the poll call, which
will continue to claim that the socket is writeable, despite it
actually isn't.

The default value for the max_ack_backlog of such a socket is 10,
meaning, this condition is likely to occur. Workarounds would be to
either decrease the socket send buffer of the sending socket or
increase the max_ack_backlog (unsigned 16-bit integer) to a more
reasonable value.

An obvious attempt at a fix would be to modify unix_writeable to take
the state of a possibly existing peer socket into account before
claiming that data can be 'send out' through a particular descriptor.

F'up2 colds
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