Spurious witness warning when destroying spin mtx

[Attilio Rao]

On Sat, Nov 24, 2012 at 3:01 PM, Attilio Rao <attilio at freebsd.org> wrote:
> On Sat, Nov 24, 2012 at 3:08 AM, Ryan Stone <rysto32 at gmail.com> wrote:
>> Today I saw a spurious witness warning for "acquiring duplicate lock of
>> same type".  The root cause is that when running mtx_destroy on a spinlock
>> that is held by the current thread, mtx_destroy calls spinlock_exit()
>> before calling WITNESS_UNLOCK, which opens up a window in which the CPU can
>> be interrupted and attempt to acquire another spinlock of the same type as
>> the one being destroyed.  This patch should fix it:
>
> I seriously wonder why right now we don't assume the lock is unheld.
> There are likely historically reasons for that, but I would like to
> know which one are those and eventually fix them out.
> FWIK, all the other locking primitives assume the lock is already
> unheld when destroying and I think it would be good to have that for
> mutexes as well.
>
> Can you please show which lock triggers the panic you saw?

Ryan,
however I'm sure this patch would introduce a major switch in our
KPI/POLA and eventually it would be a bigger work.

Your patch is certainly right and I think you should commit it for the
time being. For the long-term, maybe you would like to work on such a
patch, rather.

Attilio


-- 
Peace can only be achieved by understanding - A. Einstein