Re: Old LOR between devfs & devfsmount resurfacing?

2008/2/7, Marcel Moolenaar <xcllnt_at_mac.com>:
>
>  On Feb 7, 2008, at 10:41 AM, Attilio Rao wrote:
>
>  > 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.
>
>
> Where you see pollution, I see opportunity. In principle no code
>  should have to use these region markers, so in the few cases
>  it's needed it not only stands out, but also allows any sized
>  region, possibly covering multiple functions to be marked. That's
>  much less pollution and/or change than having to tag each and
>  every lock operation.

Yes, but what I mean is that it is enough to do something like:

WITNESS_SKIPNEXTCHECK();
lockmgr(...)

and have the same result.
Before to try something like that, I want to explore another way of
doing things as I don't like the idea to see consumers code "polluted"
 by WITNESS_* functions.

Is this clearer?

Attilio


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