Deadlock in the NFS client

[John Baldwin]

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.  What I am focusing on atm 
is testing that the deadlock is fixed (I have a way to reproduce it now).

-- 
John Baldwin