Date: Mon, 28 Jun 1999 06:47:13 +0800 From: Peter Wemm <peter@netplex.com.au> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: current@FreeBSD.ORG, mckusick@mckusick.com Subject: Re: BUF_LOCK() related panic.. Message-ID: <19990627224713.A61D781@overcee.netplex.com.au> In-Reply-To: Your message of "Sun, 27 Jun 1999 13:06:13 MST." <199906272006.NAA15499@apollo.backplane.com>
next in thread | previous in thread | raw e-mail | index | archive | help
Matthew Dillon wrote:
> :
> :But that doesn't fix the UP problem where cluster_wbuild() tries to
> :recursively re-lock a buf that the current process already owns. I have a
> :few ideas about that one though, I just don't understand the clustering
> :well enough yet to fix it.
>
> Ok, I just hit this testing the lockmgr changes.
>
> I think the problem is that cluster_wbuild's algorithm was polluted
> a little by Kirk's commit.
>
> Previously it tested for B_BUSY to determine if it could then lock it
> to include in the cluster.
Yep, I was aware of this before, but I didn't rip it out since I didn't
know what Kirk's intentions were. I'm assuming he's got his experience
with making the BSD/OS vfs reentrant in mind, so I don't want to break
anything that gets closer to that. A seperate test and then lock would
not be reentrant. (Sure, there are far bigger problems than this, but
every bit helps when we get there)
> Kirk changed this to actually attempt a lock, and then include it
> if the lock succeeded and not include it if the lock failed.
>
> The problem is that if the buffer was already locked by the same process,
> this change results in a panic instead of a simple failure to obtain the
> lock.
> The solution is to re-tool the code to use the original algorithm ( test
> the lock before trying to get it, rather then simply trying to get it ),
> but with the new locks. I do not have time today to do this but I believ
e
> I have given sufficient information for Peter, Kirk, or Alan to make the
> fix.
Actually, I think there is another set if missing BUF_KERNPROC() calls,
cluster_callback() frees buffers, so all buffers submitted with it had
better be reassigned. This is (I think) part of the problem that
cluster_wbuild() is hitting - things were supposed to have been reassigned
but are still hanging onto the current process.
> I believe there are two or three areas in Kirk's patchset where he
> replaced an explicit test with an attempt to actually gain the lock where
> this sort of panic can occur. I think Kirk was trying to optimize the
> code :-) Heh heh. Just goes to show that combining functional
> replacements with optimizations all in one go does not always work.
>
> -Matt
> Matthew Dillon
> <dillon@backplane.com>
>
> :Cheers,
> :-Peter
> :--
> :Peter Wemm - peter@FreeBSD.org; peter@yahoo-inc.com; peter@netplex.com.au
>
>
>
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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?19990627224713.A61D781>