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Subject[PATCH tip/core/rcu 5/8] rcu: only one evaluation of arg in rcu_dereference_check() unless sparse
Date
The current version of the __rcu_access_pointer(), __rcu_dereference_check(),
and __rcu_dereference_protected() macros evaluate their "p" argument
three times, not counting typeof()s. This is bad news if that argument
contains a side effect. This commit therefore evaluates this argument
only once in normal kernel builds. However, the straightforward approach
defeats sparse's RCU-pointer checking, so when __CHECKER__ is defined,
the additional pair of evaluations of the "p" argument are performed in
order to permit sparse to detect misuse of RCU-protected pointers.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
---
include/linux/rcupdate.h | 14 +++++++++++---
1 files changed, 11 insertions(+), 3 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 89414d6..03cda7b 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -310,24 +310,32 @@ extern int rcu_my_thread_group_empty(void);
* (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
* the future.
*/
+
+#ifdef __CHECKER__
+#define rcu_dereference_sparse(p, space) \
+ ((void)(((typeof(*p) space *)p) == p))
+#else /* #ifdef __CHECKER__ */
+#define rcu_dereference_sparse(p, space)
+#endif /* #else #ifdef __CHECKER__ */
+
#define __rcu_access_pointer(p, space) \
({ \
typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
- (void) (((typeof (*p) space *)p) == p); \
+ rcu_dereference_sparse(p, space); \
((typeof(*p) __force __kernel *)(_________p1)); \
})
#define __rcu_dereference_check(p, c, space) \
({ \
typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
rcu_lockdep_assert(c); \
- (void) (((typeof (*p) space *)p) == p); \
+ rcu_dereference_sparse(p, space); \
smp_read_barrier_depends(); \
((typeof(*p) __force __kernel *)(_________p1)); \
})
#define __rcu_dereference_protected(p, c, space) \
({ \
rcu_lockdep_assert(c); \
- (void) (((typeof (*p) space *)p) == p); \
+ rcu_dereference_sparse(p, space); \
((typeof(*p) __force __kernel *)(p)); \
})

--
1.7.0.6


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