Date: Thu, 14 Mar 2013 14:44:35 -0400 From: John Baldwin <jhb@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Rick Macklem <rmacklem@freebsd.org>, fs@freebsd.org Subject: Re: Deadlock in the NFS client Message-ID: <201303141444.35740.jhb@freebsd.org> In-Reply-To: <20130314172239.GL3794@kib.kiev.ua> References: <201303131356.37919.jhb@freebsd.org> <201303141057.13609.jhb@freebsd.org> <20130314172239.GL3794@kib.kiev.ua>
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On Thursday, March 14, 2013 1:22:39 pm Konstantin Belousov wrote:
> On Thu, Mar 14, 2013 at 10:57:13AM -0400, John Baldwin wrote:
> > On Thursday, March 14, 2013 5:27:28 am Konstantin Belousov wrote:
> > > On Wed, Mar 13, 2013 at 07:33:35PM -0400, Rick Macklem wrote:
> > > > John Baldwin wrote:
> > > > > I ran into a machine that had a deadlock among certain files on a
> > > > > given NFS
> > > > > mount today. I'm not sure how best to resolve it, though it seems like
> > > > > perhaps there is a bug with how the pool of nfsiod threads is managed.
> > > > > Anyway, more details on the actual hang below. This was on 8.x with
> > > > > the
> > > > > old NFS client, but I don't see anything in HEAD that would fix this.
> > > > >
> > > > > First note that the system was idle so it had dropped down to only one
> > > > > nfsiod thread.
> > > > >
> > > > Hmm, I see the problem and I'm a bit surprised it doesn't bite more often.
> > > > It seems to me that this snippet of code from nfs_asyncio() makes too
> > > > weak an assumption:
> > > > /*
> > > > * If none are free, we may already have an iod working on this mount
> > > > * point. If so, it will process our request.
> > > > */
> > > > if (!gotiod) {
> > > > if (nmp->nm_bufqiods > 0) {
> > > > NFS_DPF(ASYNCIO,
> > > > ("nfs_asyncio: %d iods are already processing mount %p\n",
> > > > nmp->nm_bufqiods, nmp));
> > > > gotiod = TRUE;
> > > > }
> > > > }
> > > > It assumes that, since an nfsiod thread is processing some buffer for the
> > > > mount, it will become available to do this one, which isn't true for your
> > > > deadlock.
> > > >
> > > > I think the simple fix would be to recode nfs_asyncio() so that
> > > > it only returns 0 if it finds an AVAILABLE nfsiod thread that it
> > > > has assigned to do the I/O, getting rid of the above. The problem
> > > > with doing this is that it may result in a lot more synchronous I/O
> > > > (nfs_asyncio() returns EIO, so the caller does the I/O). Maybe more
> > > > synchronous I/O could be avoided by allowing nfs_asyncio() to create a
> > > > new thread even if the total is above nfs_iodmax. (I think this would
> > > > require the fixed array to be replaced with a linked list and might
> > > > result in a large number of nfsiod threads.) Maybe just having a large
> > > > nfs_iodmax would be an adequate compromise?
> > > >
> > > > Does having a large # of nfsiod threads cause any serious problem for
> > > > most systems these days?
> > > >
> > > > I'd be tempted to recode nfs_asyncio() as above and then, instead
> > > > of nfs_iodmin and nfs_iodmax, I'd simply have: - a fixed number of
> > > > nfsiod threads (this could be a tunable, with the understanding that
> > > > it should be large for good performance)
> > > >
> > >
> > > I do not see how this would solve the deadlock itself. The proposal would
> > > only allow system to survive slightly longer after the deadlock appeared.
> > > And, I think that allowing the unbound amount of nfsiod threads is also
> > > fatal.
> > >
> > > The issue there is the LOR between buffer lock and vnode lock. Buffer lock
> > > always must come after the vnode lock. The problematic nfsiod thread, which
> > > locks the vnode, volatile this rule, because despite the LK_KERNPROC
> > > ownership of the buffer lock, it is the thread which de fact owns the
> > > buffer (only the thread can unlock it).
> > >
> > > A possible solution would be to pass LK_NOWAIT to nfs_nget() from the
> > > nfs_readdirplusrpc(). From my reading of the code, nfs_nget() should
> > > be capable of correctly handling the lock failure. And EBUSY would
> > > result in doit = 0, which should be fine too.
> > >
> > > It is possible that EBUSY should be reset to 0, though.
> >
> > Yes, thinking about this more, I do think the right answer is for
> > readdirplus to do this. The only question I have is if it should do
> > this always, or if it should do this only from the nfsiod thread. I
> > believe you can't get this in the non-nfsiod case.
>
> I agree that it looks as of the workaround only needed for nfsiod thread.
> On the other hand, it is not immediately obvious how to detect that
> the current thread is nfsio daemon. Probably a thread flag should be
> set.
OTOH, updating the attributes from readdir+ is only an optimization anyway, so
just having it always do LK_NOWAIT is probably ok (and simple). Currently I'm
trying to develop a test case to provoke this so I can test the fix, but no
luck on that yet.
--
John Baldwin
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