Deadlock in the NFS client

[John Baldwin]

On Monday, March 18, 2013 9:00:54 pm Rick Macklem wrote:
> John Baldwin wrote:
> > On Friday, March 15, 2013 10:03:39 pm Rick Macklem wrote:
> > > John Baldwin wrote:
> > > > 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
> > > Just fyi, ignore my comment about the second version of the patch
> > > that
> > > disables the nfsiod threads from doing readdirplus running faster.
> > > It
> > > was just that when I tested the 2nd patch, the server's caches were
> > > primed. Oops.
> > >
> > > However, sofar the minimal testing I've done has been essentially
> > > performance neutral between the unpatch and patched versions.
> > >
> > > Hopefully John has a convenient way to do some performance testing,
> > > since I won't be able to do much until the end of April.
> > 
> > Performance testing I don't really have available.
> All I've been doing are things like (assuming /mnt is an NFSv3 mount point):
> # cd /mnt
> # time ls -lR > /dev/null
> # time ls -R > /dev/null
> - for both a patched and unpatched kernel
> (Oh, and you need to keep the server's caches pretty consistent. For me
>  once I run the test once, the server caches end up primed and then
>  the times seem to be pretty consistent, but I am only using old laptops.)
> 
> Maybe you could do something like the above? (I'll try some finds too.)
> (I don't really have any clever ideas for other tests.)

I've been doing find across trees on different servers (one mounted with
rdirplus and one without).  I've compared the current behavior (blocking
lock in rdirplus + readahead) to disabling readahead for dirs as well as
just using non-blocking locks in the nfsiod case in readdir+ processing.
I also ran my tests with various concurrent number of jobs (up to 8 since
this is an 8-core machine).  The three cases were all basically the same,
and the two possible fixes were no different, so I think you can fix this
however you want.

-- 
John Baldwin