Date: Tue, 1 Aug 2006 11:22:17 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Robert Huff <roberthuff@rcn.com>, "Christian S.J. Peron" <csjp@freebsd.org> Subject: Re: LOR when booting CURRENT (ip_divert.c, PFil hook read/write mutex) [#181] Message-ID: <200608011122.18189.jhb@freebsd.org> In-Reply-To: <44CE6C0A.50009@FreeBSD.org> References: <20060726063636.GA58151@freefall.freebsd.org> <17610.836.663396.331448@jerusalem.litteratus.org> <44CE6C0A.50009@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Monday 31 July 2006 16:46, Christian S.J. Peron wrote: > Robert Huff wrote: > > Yar Tikhiy writes: > > > > > >> FWIW, the LOR still is there. I was seeing it yesterday while > >> fiddling with the ipfw and natd rc.d scripts. > >> > >> lock order reversal: > >> 1st 0xc1a36090 inp (divinp) @ /usr/src/sys/modules/ipdivert/../../netinet/ip_divert.c:350 > >> 2nd 0xc0a51918 PFil hook read/write mutex (PFil hook read/write mutex) @ /usr/src/sys/net/pfil.c:73 > >> > > > > For the record, I'm (still) getting this also. > > > > > > Robert Huff > > _______________________________________________ > > freebsd-current@freebsd.org mailing list > > http://lists.freebsd.org/mailman/listinfo/freebsd-current > > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org" > > > > > > > > This appears to be similar to the LOR associated with IPFW and ucred > based rules, I think. Although this is a lock order reversal and it > probably isn't a false positive, it should be reasonably harmless, > because the pfil hook lock is a reader lock, thus different threads can > acquire it (at this point) con-currently, presumably preventing a dead > lock from actually occurring here. > > iirc witness it not aware of the reader/writer semantics, so it makes > sense that it will be dropping a warning here. But I can look at this in > further detail when I get a bit of time. No, a LOR is a LOR. Readers vs writers don't matter for ordering reasons. Talk yourself through it and you'll see. The reason is that a writer can always block on a reader, and a reader will block if there's a writer already holding the lock. While you can end up in some situations where a LOR might not deadlock at the time if both threads involved are getting read locks, at some point a thread will need to get a write lock (otherwise you wouldn't need a lock!) and then you can get a deadlock between the thread with the write lock and a thread acquiring the locks in reverse order even if that second thread is only getting a read lock. Specifically, given mtx A, and rwlock B, while it may be safe for a thread to rlock B and lock A while another thread does lock A and rlock B w/o triggering deadlock, if a thread does lock A and then wlock B, then when another tried tries to rlock B and then lock A you will get deadlock. -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200608011122.18189.jhb>