so_rcv_sx, deadlkres, SIGSTOP

[Attilio Rao]

2010/4/4 Jilles Tjoelker <jilles at stack.nl>:
> 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.

Jilles,
may you please try this patch?:
http://www.freebsd.org/~attilio/Sandvine/deadlkres/deadlkres-blessed.diff

It does introduce a blessed list where the sx_lock can be whitelisted.
It also fixes, for long sleeps, the wrap-up of ticks object.
The obvious fallout is that deadlkres can't help much with deadlocks
for whitelisted sxlock or lockmgr but that is a fair price I think.

Thanks,
Attilio


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