[PATCH 0/2] nfsd shoudn't call mnt_want_write twice

["J. Bruce Fields" <bfields@redhat.com>]

From: "J. Bruce Fields" <bfields@redhat.com>

A fuzzer recently triggered lockdep warnings about potential sb_writers
deadlocks caused by fh_want_write, showing that we haven't been careful
to pair each fh_want_write() with an fh_drop_write().

This isn't normally a problem since fh_put() will call fh_drop_write()
for us.  And that's OK for NFSv3.  But an NFSv4 protocol fuzzer can do
weird things like call unlink twice in a compound.

So we can either make it safe to call fh_want_write() twice, or we can
fix all the callers to call fh_drop_write().

For now I think we have to do the former just to get the bug fixed.

Long term I don't know whether it's best to stay with that or to fix up
the callers.  I fixed nfsd_unlink just because it's easy, but maybe
that's pointless, it's others (like setattr) that are the complicated
ones.

J. Bruce Fields (2):
  nfsd: allow fh_want_write to be called twice
  nfsd: fh_drop_write in nfsd_unlink

 fs/nfsd/vfs.c | 8 +++++---
 fs/nfsd/vfs.h | 5 ++++-
 2 files changed, 9 insertions(+), 4 deletions(-)

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2.21.0