Date: Wed, 11 Aug 2004 22:57:12 -0700 From: John-Mark Gurney <gurney_j@resnet.uoregon.edu> To: John Baldwin <jhb@FreeBSD.org> Cc: freebsd-arch@FreeBSD.org Subject: Re: valid dup lock logic for witness Message-ID: <20040812055712.GC991@funkthat.com> In-Reply-To: <200408091026.35755.jhb@FreeBSD.org> References: <20040806224316.GB991@funkthat.com> <200408091026.35755.jhb@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
John Baldwin wrote this message on Mon, Aug 09, 2004 at 10:26 -0400: > On Friday 06 August 2004 06:43 pm, John-Mark Gurney wrote: > > I have been working on kqueue, and to support kq in kq, I need to obtain > > two kq locks (both of the same type) at the same time. Normally this can > > cause a deadlock, but using a proper lock ordering strategy, it can be > > avoided. In the kq case, I chose to aquire a kq global lock before > > acquiring multiple kq locks. (In the proc case, jhb said you aquire > > the child's before the parents.) > > > > Mutexs have the flag MTX_DUPOK that notify witness that duplicate locks > > are ok, but this can hide other problems (and in fact would have in my > > testing). > > > > I have created a patch that lets you inform witness the a duplicate lock > > is valid as long as you hold another lock. The only run time change is > > that when a duplicate lock is found, it will run through another table > > to verify it's ok before printing out the back trace. > > > > Anyone have objections to this? > > As I said on IRC, my objection to this is that there are numerous ways of > acquiring duplicate locks in a valid fashion. For kq there is a global lock > around such cases. For proc locks child processes are locked before parents. > The problem is that there is not a single way of doing this, so if you want > WITNESS to check all of these, you will have to add lots of special case > one-off hacks to WITNESS making it even more obtuse and slow. Perhaps > something that might be feasible is to provide some sort of way for other > parts of the kernel to register a duplicate check function for a given lock > type. This would let you keep the code doing the duplicate check closer to > the code using the locks for one thing and would avoid adding N hacks to > witness for the various different dup lock checks. How about that, but making the dup lock ok w/ a signle parent lock a generic function. I would imagine there are a finite number of ways to solve duplicate locks, and they will end up being shared between different subsystems. -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20040812055712.GC991>