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Date:	Fri, 30 Apr 2010 17:26:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/urgent 10/10] vfs: fix RCU-lockdep false positive due to /proc access

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.

Located-by: Miles Lane <miles.lane@...il.com>
Located-by: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/fdtable.h |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 013dc52..e4a6d31 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 || \
+			       thread_group_empty(current)))
 
 #define files_fdtable(files) \
 		(rcu_dereference_check_fdtable((files), (files)->fdt))
-- 
1.7.0

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