Date: Mon, 26 Jan 2009 10:05:21 -0800 From: Julian Elischer <julian@elischer.org> To: John Baldwin <jhb@freebsd.org> Cc: arch@freebsd.org, Alfred Perlstein <alfred@freebsd.org>, Kip Macy <kmacy@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: need for another mutex type/flag? Message-ID: <497DFB61.1010602@elischer.org> In-Reply-To: <200901260936.18232.jhb@freebsd.org> References: <497BA91D.805@elischer.org> <497D5DF8.8000706@elischer.org> <20090126105140.GL5889@elvis.mu.org> <200901260936.18232.jhb@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
John Baldwin wrote: > On Monday 26 January 2009 5:51:40 am Alfred Perlstein wrote: >> * Julian Elischer <julian@elischer.org> [090125 22:53] wrote: >>> Alfred Perlstein wrote: >>>> Jeff, I think that Julian really wants to prevent a sleep inside >>>> his context. Right now, I think we only check for mutexes held >>>> before a sleep that arne't sleepable. It might make sense to allow >>>> one to just mark a thread non-sleepable even though no mutex is >>>> held. >>>> >>>> Julian, is that right? >>> basically, though I don't know the details of implementation.. >>> I just know that mutexes per se aren't bad for netgraph but >>> that node authors need some guidance on how to use them and >>> some way to prove to them when they do the wrong thing. >> The way to add the assertion you want would be to keep a count >> inside of the thread structure "td_nosleep", set to 0 at thread >> creation, then you can do this: >> >> TD_SLEEP_NO(td); /* td->td_nosleep++ */ >> call_some_untrusted_code(); >> TD_SLEEP_OK(td); /* td->td_nosleep-- */ >> >> Then add this to subr_witness.c:witness_warn(): >> >> if (flags & WARN_SLEEPOK && td->td_nosleep != 0) { >> printf("Sleeping in unsleepable context.\n"); >> n++; /* this variable is local to witness_warn() >> and triggers an ASSERT at the end */ >> } >> >> I could have sworn we already had such a feature, but it appears >> that it's only accessable if you're holding a lock, if you added >> this counter, then you could catch sleeps without needing a lock >> held. > > We have this feature already for sleeping, but I think Julian isn't worried > about sleeping (i.e. *sleep() or cv_*wait*()), but wants to prevent the code > from acquiring any other locks. It's easy to add a MTX_LEAF, I'm just not > sure if we really want to micro-manage the code that much. WITNESS will > already catch any LORs, and if they are acquiring a rarely-contested lock > then they aren't going to back up traffic in the common case. > maybe what I want is to be able to label a lock as "fleeting" By which I mean that the work that would be done while holding this lock would be fleeting (momentary) in nature. An example f a fleeting lock would be something that gains the lock in order to safely switch two pointers. no malloc is required and nothing is going to take a long time. locks that are NOT momentary include holding the process list lock while allocating a large buffer (series of them) and dumping the contents of all processes and things like that. one might almost say that a fleeting lock might be gotten while holding another fleeting lock, but that is pushing it for me..
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?497DFB61.1010602>