unix: new implementation of unix/stream & unix/seqpacket
Provide protocol specific pr_sosend and pr_soreceive for PF_UNIX
SOCK_STREAM sockets and implement SOCK_SEQPACKET sockets as an extension
of SOCK_STREAM. The change meets three goals: get rid of unix(4) specific
stuff in the generic socket code, provide a faster and robust unix/stream
sockets and bring unix/seqpacket much closer to specification. Highlights
follow:
- The send buffer now is truly bypassed. Previously it was always empty,
but the send(2) still needed to acquire its lock and do a variety of
tricks to be woken up in the right time while sleeping on it. Now the
only two things we care about in the send buffer is the I/O sx(9) lock
that serializes operations and value of so_snd.sb_hiwat, which we can read
without obtaining a lock. The sleep of a send(2) happens on the mutex of
the receive buffer of the peer. A bulk send/recv of data with large
socket buffers will make both syscalls just bounce between owning the
receive buffer lock and copyin(9)/copyout(9), no other locks would be
involved.
- The implementation uses new mchain structure to manipulate mbuf chains.
Note that this required converting to mchain two functions that are shared
with unix/dgram: unp_internalize() and unp_addsockcred() as well as adding
a new shared one uipc_process_kernel_mbuf(). This induces some non-
functional changes in the unix/dgram code as well. There is a space for
improvement here, as right now it is a mix of mchain and manually managed
mbuf chains.
- unix/seqpacket previously marked as PR_ADDR & PR_ATOMIC and thus treated
as a datagram socket by the generic socket code, now becomes a true stream
socket with record markers.
- unix/stream loses the sendfile(2) support. This can be brought back,
but requires some work. Let's first see if there is any interest in this
feature, except purely academical.
Reviewed by: markj, tuexen
Differential Revision: https://reviews.freebsd.org/D44151