kernel: page fault in unp_pcb_owned_lock2_slowpath

[Mark Johnston]

On Thu, Oct 08, 2020 at 09:58:09PM +0000, Leverett, Bruce wrote:
> In 12.1, we are seeing a page fault in unp_pcb_owned_lock2_slowpath, while trying to lock unp2.  Examination of the crash dump shows that unp2's reference count is down to zero, which it shouldn't be, since the function took a reference on it before unlocking unp.
> 
> Could this be a bug that has been fixed in recent versions?  I would look into upgrading, or back-porting the fix, if a fix is known.

I recently fixed a few issues with the unix domain socket locking code.
The commits were merged to stable/12 in r366488.  There's a few earlier
fixes in uipc_usrreq.c that were merged after 12.1, so you might have
luck backporting those as well.  I'm not sure what the specific bug is
in your case; a backtrace at least might be enough to pinpoint it.