Commit-ID: b826565aaf8809df146666c03d1acbb7febbd13e Gitweb: http://git.kernel.org/tip/b826565aaf8809df146666c03d1acbb7febbd13e Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Mon, 2 Feb 2015 11:46:33 -0800 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Tue, 3 Mar 2015 11:16:47 -0800 rcu: Reverse rcu_dereference_check() conditions The rcu_dereference_check() family of primitives evaluates the RCU lockdep expression first, and only then evaluates the expression passed in. This works fine normally, but can potentially fail in environments (such as NMI handlers) where lockdep cannot be invoked. The problem is that even if the expression passed in is "1", the compiler would need to prove that the RCU lockdep expression (rcu_read_lock_held(), for example) is free of side effects in order to be able to elide it. Given that rcu_read_lock_held() is sometimes separately compiled, the compiler cannot always use this optimization. This commit therefore reverse the order of evaluation, so that the expression passed in is evaluated first, and the RCU lockdep expression is evaluated only if the passed-in expression evaluated to false, courtesy of the C-language short-circuit boolean evaluation rules. This compells the compiler to forego executing the RCU lockdep expression in cases where the passed-in expression evaluates to "1" at compile time, so that (for example) rcu_dereference_raw() can be guaranteed to execute safely within an NMI handler. Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> --- include/linux/rcupdate.h | 6 +++--- include/linux/srcu.h | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 70b896e..416ae28 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -729,7 +729,7 @@ static inline void rcu_preempt_sleep_check(void) * annotated as __rcu. */ #define rcu_dereference_check(p, c) \ - __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu) + __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu) /** * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking @@ -739,7 +739,7 @@ static inline void rcu_preempt_sleep_check(void) * This is the RCU-bh counterpart to rcu_dereference_check(). */ #define rcu_dereference_bh_check(p, c) \ - __rcu_dereference_check((p), rcu_read_lock_bh_held() || (c), __rcu) + __rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu) /** * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking @@ -749,7 +749,7 @@ static inline void rcu_preempt_sleep_check(void) * This is the RCU-sched counterpart to rcu_dereference_check(). */ #define rcu_dereference_sched_check(p, c) \ - __rcu_dereference_check((p), rcu_read_lock_sched_held() || (c), \ + __rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \ __rcu) #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/ diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 9cfd962..bdeb456 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -182,7 +182,7 @@ static inline int srcu_read_lock_held(struct srcu_struct *sp) * lockdep_is_held() calls. */ #define srcu_dereference_check(p, sp, c) \ - __rcu_dereference_check((p), srcu_read_lock_held(sp) || (c), __rcu) + __rcu_dereference_check((p), (c) || srcu_read_lock_held(sp), __rcu) /** * srcu_dereference - fetch SRCU-protected pointer for later dereferencing -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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