Re: Hang due to nfs letting tasks freeze with locked inodes

[Jeff Layton <jlayton@redhat.com>]

On Wed, 2016-07-06 at 12:46 -0500, Seth Forshee wrote:
> We're seeing a hang when freezing a container with an nfs bind mount while
> running iozone. Two iozone processes were hung with this stack trace.
> 
>  [] schedule+0x35/0x80
>  [] schedule_preempt_disabled+0xe/0x10
>  [] __mutex_lock_slowpath+0xb9/0x130
>  [] mutex_lock+0x1f/0x30
>  [] do_unlinkat+0x12b/0x2d0
>  [] SyS_unlink+0x16/0x20
>  [] entry_SYSCALL_64_fastpath+0x16/0x71
> 
> This seems to be due to another iozone thread frozen during unlink with
> this stack trace:
> 
>  [] __refrigerator+0x7a/0x140
>  [] nfs4_handle_exception+0x118/0x130 [nfsv4]
>  [] nfs4_proc_remove+0x7d/0xf0 [nfsv4]
>  [] nfs_unlink+0x149/0x350 [nfs]
>  [] vfs_unlink+0xf1/0x1a0
>  [] do_unlinkat+0x279/0x2d0
>  [] SyS_unlink+0x16/0x20
>  [] entry_SYSCALL_64_fastpath+0x16/0x71
> 
> Since nfs is allowing the thread to be frozen with the inode locked it's
> preventing other threads trying to lock the same inode from freezing. It
> seems like a bad idea for nfs to be doing this.
> 

Yeah, known problem. Not a simple one to fix though.

> Can nfs do something different here to prevent this? Maybe use a
> non-freezable sleep and let the operation complete, or else abort the
> operation and return ERESTARTSYS?

The problem with letting the op complete is that often by the time you
get to the point of trying to freeze processes, the network interfaces
are already shut down. So the operation you're waiting on might never
complete. Stuff like suspend operations on your laptop fail, leading to
fun bug reports like: "Oh, my laptop burned to crisp inside my bag
because the suspend never completed."

You could (in principle) return something like -ERESTARTSYS iff the
call has not yet been transmitted. If it has already been transmitted,
then you might end up sending the call a second time (but not as an RPC
retransmission of course). If that call was non-idempotent then you end
up with all of _those_ sorts of problems.

Also, -ERESTARTSYS is not quite right as it doesn't always cause the
call to be restarted. It depends on the syscall. I think this would
probably need some other sort of syscall-restart machinery plumbed in.

-- 
Jeff Layton <jlayton@redhat.com>