so_rcv_sx, deadlkres, SIGSTOP

[Jilles Tjoelker]

The SX locks so_snd_sx and so_rcv_sx can be legitimately held for
arbitrary amounts of time, while waiting until a send or recv is
possible. Any other threads wanting to send/recv on the socket will
sleep interruptibly on the corresponding SX. If deadlkres is activated,
it may detect a deadlock even though there is no problem.

If a SIGSTOP or similar comes in while waiting until send/recv is
possible, the SX is held across the suspension and noone will be able to
send/recv on the socket until the process is resumed. On the other hand
a thread waiting for the SX can be suspended without harm. Adding PBDRY
to various sleeps may help but may also introduce other problems
(SIGSTOP disturbing the functioning of the process more, possibly with
stuff like SO_RCVTIMEO).

Example (using the fact that fifos are implemented using sockets):

term1% mkfifo testfifo
term1% cat >testfifo

term2% cat testfifo

term3% cat testfifo

Letting this sit for half an hour will trigger deadlkres.

If the reader in term2 (started first) is suspended, the reader in term3
will not get any input written to the fifo.

-- 
Jilles Tjoelker