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vfs: fix RCU-lockdep false positive due to /proc 

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Paul E. McKenney 2010-07-20 13:24:34 -07:00 committed by Linus Torvalds
parent 9d51a6b248
commit 844b9a8707
1 changed files with 2 additions and 1 deletions
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3
include/linux/fdtable.h
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@ -61,7 +61,8 @@ struct files_struct {
(rcu_dereference_check((fdtfd), \ (rcu_dereference_check((fdtfd), \
rcu_read_lock_held() || \ rcu_read_lock_held() || \
lockdep_is_held(&(files)->file_lock) || \ 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) \ #define files_fdtable(files) \
(rcu_dereference_check_fdtable((files), (files)->fdt)) (rcu_dereference_check_fdtable((files), (files)->fdt))