Spurious witness warning when destroying spin mtx

Attilio Rao attilio at freebsd.org
Sat Nov 24 15:01:42 UTC 2012

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?


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