Old LOR between devfs & devfsmount resurfacing?

Attilio Rao attilio at freebsd.org
Thu Feb 7 18:41:46 UTC 2008


2008/2/7, Marcel Moolenaar <xcllnt at mac.com>:
> On Feb 7, 2008, at 6:11 AM, Attilio Rao wrote:
>
>  >>>>>>>> Correct lock order is devfs vnode -> devfs mount sx lock. When
>  >>>>>>>> allocating new devfs vnode, see devfs_allocv(), the newly
>  >>>>>>>> created
>  >>>>>>>> vnode is locked while devfs mount lock already held (see line
>  >>>>>>>> 250 of
>  >>>>>>>> fs/devfs/devfs_vnops.c). Nonetheless, this cannot cause
>  >>>>>>>> deadlock since
>  >>>>>>>> no other thread can find the new vnode, and thus perform the
>  >>>>>>>> other lock
>  >>>>>>>> order for this vnode lock.
>  >>>>>>>>
>  >>>>>>>> The fix is to shut the witness in this particular case.
>  >>>>>>>> Attilio, how to
>  >>>>>>>> do this ?
>  >>>>>>>
>  >>>>>>> Just add LK_NOWITNESS for one of the lock involved in the
>  >>>>>>> lockinit().
>  >>>>>>
>  >>>>>>
>  >>>>>> Then, we loss the useful reports of the actual LORs later,
>  >>>>>> isn't it ?
>  >>>>>
>  >>>>> Another solution would be to rewamp BLESSING option which allow to
>  >>>>> 'bless' some LORs.
>  >>>>> jhb and me, btw, didn't want to enable it because it could lead
>  >>>>> some
>  >>>>> less experienced developer to hide LORs under this label and
>  >>>>> this is
>  >>>>> something we want to avoid.
>  >>>>
>  >>>>
>  >>>> This LOR shall not be ignored globally. When real, it caused the
>  >>>> easily
>  >>>> reproducable lockup of the machine.
>  >>>>
>  >>>> It would be better to introduce some lockmgr flag to ignore
>  >>>> _this_ locking.
>  >>>
>  >>> flag to pass where?
>  >> To the lockmgr itself at the point of aquisition, like
>  >>     lockmgr(&lk, LK_EXCLUSIVE | LK_INTERLOCK | LK_NOWARN,
>  >> &interlk, ...);
>  >
>  > No, I really want a general WITNESS support for this (as I also think
>  > that having something more fine grained than BLESSING will break all
>  > concerns jhb and me are considering now).
>  > A simple way to do it would mean hard-coding file and line in a
>  > witness table. While file is ok, line makes trouble so we should find
>  > an alternative way to do this. Otherwise we can consider skiping
>  > checks for a whole function, this should be not so difficult to
>  > achive.
>  >
>  > I need to think more about this.
>
>
> What about a linker set that lists file regions (based on line number).
>  If you want to exclude a particular lock from WITNESS you can do
>  something like this:
>         WITNESS_REGION_START(function)
>         lockmgr(...)
>         WITNESS_REGION_END
>
>  The WITNESS_REGION_START and WITNESS_WITNESS_END together create a
>  region in the linker set and witness can check if a lock operation
>  falls within that region. If yes, we can make it do something special
>  by given the _START and/or _END a function pointer or we can make it
>  ignore the operation by passing NULL or something.
>
>  You can safely use file & line numbers in this case. Something along
>  those lines...
>
>  Thoughts?

Really, if I wanted to pollute consumers code I would have use a lot
of simpler ideas.
I'd like strongly to maintain WITNESS_* namespace usage only in
locking primitives.

Attilio


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


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