From owner-freebsd-current Sun Jun 27 3:37:12 1999 Delivered-To: freebsd-current@freebsd.org Received: from overcee.netplex.com.au (overcee.netplex.com.au [202.12.86.7]) by hub.freebsd.org (Postfix) with ESMTP id 8D70614CA3 for ; Sun, 27 Jun 1999 03:36:52 -0700 (PDT) (envelope-from peter@netplex.com.au) Received: from netplex.com.au (localhost [127.0.0.1]) by overcee.netplex.com.au (Postfix) with ESMTP id CEB4B81; Sun, 27 Jun 1999 18:36:49 +0800 (WST) (envelope-from peter@netplex.com.au) X-Mailer: exmh version 2.0.2 2/24/98 To: Doug Rabson Cc: Matthew Dillon , current@freebsd.org, mckusick@mckusick.com Subject: Re: BUF_LOCK() related panic.. In-reply-to: Your message of "Sun, 27 Jun 1999 11:35:44 +0100." Date: Sun, 27 Jun 1999 18:36:49 +0800 From: Peter Wemm Message-Id: <19990627103649.CEB4B81@overcee.netplex.com.au> Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG Doug Rabson wrote: > On Sun, 27 Jun 1999, Peter Wemm wrote: > > > Doug Rabson wrote: > > > On Sun, 27 Jun 1999, Peter Wemm wrote: > > > > > > > Matthew Dillon wrote: > > > > > Ah, yes, some of us were just discussing this in a small mailing list > > . > > > > > Hopefully Kirk will pick up on it soon. Ah well.. someone else g ets > > to b > > > > e > > > > > the brunt of it for a change :-). Kirk doesn't have an SMP box s o he > > > > > didn't see the bug. > > > > > > > > > > I have tentitively tracked the problem down to the apparent inabi lity > > of > > > > > lockmgr() locks to function from interrupts, even when used in a > > > > > non-blocking manner, due to the simplelock's it uses internally. The > > > > > new buffer cache code Kirk committed switched from B_BUSY (manual ly > > > > > implemented locks) to lockmgr() locks. I think what is going on is > > > > > that mainline code is getting a simplelock and then an interrupt is > > > > > coming along and also trying to get the same lock, but I can't be sur > > e > > > > > because my DDB backtraces are somewhat munged. > > > > > > > > In this case, it was just a programming error.. The key to remember is tha > > t > > > > the simplelocks are used to protect the state of the complex lock, they are > > > > not the lock themselves. lockmgr() holds the interlock while gaining o r > > > > removing references etc and then frees the simplelock so that it can sl eep > > > > if required etc. The actual implementation of the simplelock routines > > > > is interrupt safe (and has to be). > > > > > > The simple_lock* macros don't seem to use the interrupt safe versions > > > (ss_lock etc). What happens if an interrupt is recieved after gaining > > > buftimelock and the interrupt routine also tries to call BUF_LOCK? > > > > Good question, but I'm not sure ss_lock is what's needed either since that > > does a cli for the duration of the simplelock being held.. > > > > I think the BUF_*() inlines need internal splbio() protection since > > a biodone() can be called from the tail end of an interrupt, and that *does * > > try and get a simplelock during a BUF_UNLOCK()... (and BUF_REFCNT()). > > In the long term, we probably need an spl-aware simplelock or maybe the > cunning no-cost interrupt thread scheme which BSDi are using. Yes. I will also note that this approach is what John Dyson wanted to do for the G2 kernel. Cheers, -Peter -- Peter Wemm - peter@FreeBSD.org; peter@yahoo-inc.com; peter@netplex.com.au To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message