Date: Thu, 7 Feb 2008 19:41:44 +0100 From: "Attilio Rao" <attilio@freebsd.org> To: "Marcel Moolenaar" <xcllnt@mac.com> Cc: Kostik Belousov <kostikbel@gmail.com>, current@freebsd.org Subject: Re: Old LOR between devfs & devfsmount resurfacing? Message-ID: <3bbf2fe10802071041j6d2748f9n16c28541da1e0430@mail.gmail.com> In-Reply-To: <4CB73483-B3BB-4819-B3B5-5F349D16C6F7@mac.com> References: <B269B28B-C66E-4AC6-A4D9-FBA378466F89@juniper.net> <20080207045015.GW57756@deviant.kiev.zoral.com.ua> <3bbf2fe10802070216idd5206ey7a66c0873311e66c@mail.gmail.com> <20080207104354.GY57756@deviant.kiev.zoral.com.ua> <3bbf2fe10802070304r29cb8d2u1210fe285c917424@mail.gmail.com> <20080207110901.GZ57756@deviant.kiev.zoral.com.ua> <3bbf2fe10802070421m3a3152a3m6c9aa67d649107e4@mail.gmail.com> <20080207125252.GC57756@deviant.kiev.zoral.com.ua> <3bbf2fe10802070611v6c7714b5y18bef10d586944c4@mail.gmail.com> <4CB73483-B3BB-4819-B3B5-5F349D16C6F7@mac.com>
next in thread | previous in thread | raw e-mail | index | archive | help
2008/2/7, Marcel Moolenaar <xcllnt@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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?3bbf2fe10802071041j6d2748f9n16c28541da1e0430>