Re: [2.6.31-rc7] NFS4 client manager kthread spinning...

[Trond Myklebust]

On Mon, 2009-08-24 at 16:18 +0100, Daniel J Blueman wrote:
> Hi Trond,
> 
> On Mon, Aug 17, 2009 at 2:53 PM, Daniel J
> Blueman<daniel.blueman@xxxxxxxxx> wrote:
> > Hi Trond,
> >
> > On Mon, Aug 17, 2009 at 2:12 PM, Trond
> > Myklebust<Trond.Myklebust@xxxxxxxxxx> wrote:
> >> On Sun, 2009-08-16 at 23:40 +0100, Daniel J Blueman wrote:
> >>> After losing and regaining ethernet link a few times with 2.6.31-rc5
> >>> [1], I've hit an oops in the NFS4 client manager kthread [2] on my
> >>> client with NFS4 homedir mount.
> >>>
> >>> Do you have a frequent test-case for when the client's manager kthread
> >>> gets invoked (with and without succeeding callbacks, due to eg a
> >>> firewall)? Server here is unpatched 2.6.30-rc6; I recall seeing
> >>> problems when the manager kthread gets invoked, across quite a few
> >>> kernel releases, just wasn't lucky enough to catch an oops.
> >>>
> >>> Oppsing in allow_signal() suggests task state corruption perhaps? I'm
> >>> downloading the debug kernel to match up the disassembly and line
> >>> numbers, if that helps? This time, the client had no firewall (but
> >>> have seen other issues when the callback has failed due to the
> >>> firewall).
> >>
> >> Those aren't Oopses. They are 'soft lockup' warnings. Basically, they're
> >> saying that the CPU is getting stuck waiting for a spin lock or a mutex.
> >>
> >> In this case, it is probably the fact that the state manager is going
> >> nuts trying to recover, while the connection to the server keeps coming
> >> up and going down.
> >>
> >> What does 'netstat -t' say when you get into this situation?
> >
> > Whoops; it's true the stack-trace comes from the soft-lockup detector.
> >
> > There was a single 200s link excursion, but the client didn't recover
> > as locks are held and never released it seems; I observe the
> > '192.168.1.250-m' NFS4 manager kthread being created and not going
> > away, despite IP connectivity with the server being fine after.
> >
> > I'll reproduce it with stock 2.6.31-rc6 on the client and get 'netstat
> > -t' output.
> 
> (subject line updated)
> 
> After further analysis, I see that NFS services do correctly recover
> after the link excursion, however we see:
>  - link is restored
>  - the manager kthread gets created, does some work
>  - we see lock reclamation fail [1]
>  - after a short while, NFS read()s continue, all is good
>  - the manager kthread spins indefinitely [2, 3] on (struct
> rpc_wait_queue)queue->lock with spin_lock_bh() [see rpc_wake_up]
> 
> This seems reproducible with various kernel debugging enabled (perhaps
> suggesting use-after-free via the lock being reinitialised/poisoned?).
> 
> Let me know if anything else may help track this down (config, stack
> frame resolution etc). I'll take a deeper look if I get time in a
> couple of weeks, but alas it may be after 2.6.31 is released. NFS+RPC
> debugging (taken at a different time than [1]) at
> http://quora.org/hive/nfs-manager-spin.bz2 .

I think I've found the bug. Does the following patch fix it for you?

Cheers
  Trond
----------------------------------------------------------------------
From: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
NFSv4: Fix an infinite looping problem with the nfs4_state_manager

Commit 76db6d9500caeaa774a3e32a997eba30bbdc176b (nfs41: add session setup
to the state manager) introduces an infinite loop possibility in the NFSv4
state manager. By first checking nfs4_has_session() before clearing the
NFS4CLNT_SESSION_SETUP flag, it allows for a situation where someone sets
that flag, but it never gets cleared, and so the state manager loops.

In fact commit c3fad1b1aaf850bf692642642ace7cd0d64af0a3 (nfs41: add session
reset to state manager) causes this to happen every time we get a network
partition error.

Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
---

 fs/nfs/nfs4state.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)


diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
index 65ca8c1..1434080 100644
--- a/fs/nfs/nfs4state.c
+++ b/fs/nfs/nfs4state.c
@@ -1250,8 +1250,8 @@ static void nfs4_state_manager(struct nfs_client *clp)
 				continue;
 		}
 		/* Initialize or reset the session */
-		if (nfs4_has_session(clp) &&
-		   test_and_clear_bit(NFS4CLNT_SESSION_SETUP, &clp->cl_state)) {
+		if (test_and_clear_bit(NFS4CLNT_SESSION_SETUP, &clp->cl_state)
+		   && nfs4_has_session(clp)) {
 			if (clp->cl_cons_state == NFS_CS_SESSION_INITING)
 				status = nfs4_initialize_session(clp);
 			else

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com
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