NFSroot hangs with bad unlock balance in Linux next

[Al Viro]

On Sun, May 08, 2016 at 03:16:29PM +0100, Al Viro wrote:

> Very strange.  We grab that rwsem at the entry into nfs_call_unlink()
> and then either release it there and return or call nfs_do_call_unlink().
> Which arranges for eventual call of nfs_async_unlink_release() (via
> ->rpc_release); nfs_async_unlink_release() releases the rwsem.  Nobody else
> releases it (on the read side, that is).
> 
> The only kinda-sorta possibility I see here is that the inode we are
> unlocking in that nfs_async_unlink_release() is not the one we'd locked
> in nfs_call_unlink() that has lead to it.  That really shouldn't happen,
> though...  Just to verify whether that's what we are hitting, could you
> try to reproduce that thing with the patch below on top of -next and see
> if it triggers any of those WARN_ON?

D'oh...  Lockdep warnings are easy to trigger (and, AFAICS, bogus).
up_read/down_read in fs/nfs/unlink.c should be replaced with
up_read_non_owner/down_read_non_owner, lest the lockdep gets confused.
Hangs are different - I've no idea what's triggering those.  I've seen
something similar on that -next, but not on work.lookups.

The joy of bisecting -next...  <a couple of hours later>
9317bb69824ec8d078b0b786b6971aedb0af3d4f is the first bad commit
commit 9317bb69824ec8d078b0b786b6971aedb0af3d4f
Author: Eric Dumazet <edumazet at google.com>
Date:   Mon Apr 25 10:39:32 2016 -0700

    net: SOCKWQ_ASYNC_NOSPACE optimizations

Reverting changes to sk_set_bit/sk_clear_bit gets rid of the hangs.  Plain
revert gives a conflict, since there had been additional change in
"net: SOCKWQ_ASYNC_WAITDATA optimizations"; removing both fixed the hangs.

Note that hangs appear without any fs/nfs/unlink.c modifications being
there.  When the hang happens it affects NFS traffic; ssh session still
works fine until it steps on a filesystem operation on NFS (i.e. you
can use builtins, access procfs, etc.)