[PATCH] vfs: fix RCU-lockdep false positive due to /proc access

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

If a single-threaded process does a file-descriptor operation, and
some other process accesses that same file descriptor via /proc,
the current rcu_dereference_check_fdtable() can give a false-positive
RCU-lockdep splat due to the reference count being increased by the
/proc access after the reference-count check in fget_light() but before
the check in rcu_dereference_check_fdtable().

This commit prevents this false positive by checking for a single-threaded
process.  To avoid #include hell, this commit uses the wrapper for
thread_group_empty(current) defined by rcu_my_thread_group_empty()
provided in a separate commit.

Located-by: Miles Lane <miles.lane@gmail.com>
Located-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
---
 fdtable.h |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 013dc52..d147461 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -61,7 +61,8 @@ struct files_struct {
 	(rcu_dereference_check((fdtfd), \
 			       rcu_read_lock_held() || \
 			       lockdep_is_held(&(files)->file_lock) || \
-			       atomic_read(&(files)->count) == 1))
+			       atomic_read(&(files)->count) == 1 || \
+			       rcu_my_thread_group_empty()))
 
 #define files_fdtable(files) \
 		(rcu_dereference_check_fdtable((files), (files)->fdt))