On 12 Jun 2013, Al Viro told this: > On Mon, Jun 10, 2013 at 06:42:49PM +0100, Nix wrote: >> Yes, my shutdown scripts are panicking the kernel again! They're not >> causing filesystem corruption this time, but it's still fs-related. >> >> Here's the 3.9.5 panic, seen on an x86-32 NFS client using NFSv3: NFSv4 >> was compiled in but not used. This happened when processes whose >> current directory was on one of those NFS-mounted filesystems were being >> killed, after it had been lazy-umounted (so by this point its cwd was in >> a disconnected mount point). >> >> [ 251.246800] BUG: unable to handle kernel NULL pointer dereference at 00000004 >> [ 251.256556] IP: [<c01739f6>] path_init+0xc7/0x27f >> [ 251.256556] *pde = 00000000 >> [ 251.256556] Oops: 0000 [#1] >> [ 251.256556] Pid: 748, comm: su Not tainted 3.9.5+ #1 >> [ 251.256556] EIP: 0060:[<c01739f6>] EFLAGS: 00010246 CPU: 0 >> [ 251.256556] EIP is at path_init+0xc7/0x27f > > Apparently that's set_root_rcu() with current->fs being NULL. Which comes from > AF_UNIX connect done by some twisted call chain in context of hell knows what. It's all NFS's fault! >> [ 251.256556] [<c02ef8da>] ? unix_stream_connect+0xe1/0x2f7 >> [ 251.256556] [<c026a14d>] ? kernel_connect+0x10/0x14 >> [ 251.256556] [<c031ecb1>] ? xs_local_connect+0x108/0x181 >> [ 251.256556] [<c031c83b>] ? xprt_connect+0xcd/0xd1 At this point, we have a sibcall to call_connect() I think. The RPC task of discourse happens to be local, and as the relevant comment says * We want the AF_LOCAL connect to be resolved in the * filesystem namespace of the process making the rpc * call. Thus we connect synchronously. Probably this should be doing this only if said namespace isn't disconnected and going away... >> [ 251.256556] [<c031fd1b>] ? __rpc_execute+0x5b/0x156 >> [ 251.256556] [<c0128ac2>] ? wake_up_bit+0xb/0x19 >> [ 251.256556] [<c031b83d>] ? rpc_run_task+0x55/0x5a >> [ 251.256556] [<c031b8bc>] ? rpc_call_sync+0x7a/0x8d >> [ 251.256556] [<c0325127>] ? rpcb_register_call+0x11/0x20 >> [ 251.256556] [<c032548a>] ? rpcb_v4_register+0x87/0xf6 This is happening because of this code in net/sunrpc/svc.c (and, indeed, I am running rpcbind, like everyone should be these days): /* * If user space is running rpcbind, it should take the v4 UNSET * and clear everything for this [program, version]. If user space * is running portmap, it will reject the v4 UNSET, but won't have * any "inet6" entries anyway. So a PMAP_UNSET should be sufficient * in this case to clear all existing entries for [program, version]. */ static void __svc_unregister(struct net *net, const u32 program, const u32 version, const char *progname) { int error; error = rpcb_v4_register(net, program, version, NULL, ""); /* * User space didn't support rpcbind v4, so retry this * request with the legacy rpcbind v2 protocol. */ if (error == -EPROTONOSUPPORT) error = rpcb_register(net, program, version, 0, 0); Ah yes, because what unregister should do is *register* something. That's clear as mud :) > Why is it done in essentially random process context, anyway? There's such thing > as chroot, after all, which would screw that sucker as hard as NULL ->fs, but in > a less visible way... I don't think it is a random process context. It's all intentionally done in the context of the process which is the last to close that filesystem, as part of the process of tearing it down -- but it looks like the NFS svcrpc connection code isn't expecting to be called in that situation. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html