On 2/16/12 12:39 AM, Bruce Evans wrote:
On Thu, 16 Feb 2012, Eitan Adler wrote:
Log:
Add a timestamp to the msgbuf output in order to determine when when
messages were printed.
This can be enabled with the kern.msgbuf_show_timestamp sysctl
Apart from being fundamentally broken, this adds lots of bloat and
style bugs. The msgbuf is a very low-level interface, and was
careful not to do stuff like this. I'm still waiting for the previous
round of breakage of it, that replaces careful atomic ops by spinlocks,
to be backed out. Code at this level cannot use any normal locking,
and used to be carefully written to not do so. The spinlocks break it,
for example, if there is a trap while holding the lock and the trap
handler wants to use the message buffer. Interrupts are possible
too, but they are disabled on the same CPU for technical reasons, so
they cannot cause deadlock here. The main possiblities for traps are
NMIs. The NMI handler shouldn't call printf, but perhaps it does.
It might cause a panic. Then panic can only print by blowing open
the locks. It might be a STOP IPI. The stop function uses printf
to diagnose other blockages, at least with certain options. Anyway
printf must never block endlessly, so msgbuf functions must not never
block endlessly either, so neither can use any normal locking, since
normal locking can easily block endlessly in deadlock conditions, by
the definition of deadlock.
Bruce, this is a good example of a legitimate gripe going un-noticed
because
you didn't shout loud enough at the right time, at the right people.
It's been about 20 years since we started working on this but I've finally
come to the point of saying that we need you to do more when you see
problems.
object officially if you think things should be backed out!
your reasons here seem sound, so it's hard to see why you haven't been
more
public about it.
Julian
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