Re: [9fans] sleep/wakeup bug?

[erik quanstrom <quanstro@quanstro.net>]

> it doesn't.  the scenario in my first email has exactly one
> sleep and one wakeup.

alright.  i do see it now, and this does look like
a source of many gnarly, and hard to find bugs.
pardon me for being so thick.

why doesn't wakeup take a function and a pointer
analogous to sleep?  that would seem to lead one
in proper usage.  or is there a more subtle hole that
i also don't see in doing it that way?

perhaps it's silly to put a thumb in the dyke, but it
might be better than relying on proper fixes for everything
appearing overnight.

in the case of this particular bug, i have at least 40µs
grace and the change has held up for 12 hrs, where i
could crash the machine in <5s before.

> as i said originally, it's simply not 1:1.
> if you need 1:1, you need a semaphore.

unfortunately, we don't have those in the kernel.

> p.s. not relevant to your "only one sleep and one wakeup"
> constraint, but that last scenario also means that if you
> are doing repeated sleep + wakeup on a single r, that pending
> wakeup call left over on cpu2 might not happen until cpu1 has
> gone back to sleep (a second time).  that is, the first wakeup
> can wake the second sleep, intending to wake the first sleep.
> so in general you have to handle the case where sleep
> wakes up for no good reason.  it doesn't happen all the time,
> but it does happen.

this is probablly a bug lurking in many things.

- erik