From: Paul E. McKenney Date: Thu, 20 Mar 2008 00:00:57 +0000 (-0700) Subject: rcu: fix misplaced mb() in rcu_enter/exit_nohz() X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=ae66be9b71b12f16b84129860d06bbfe37fbec51;p=GitHub%2Fexynos8895%2Fandroid_kernel_samsung_universal8895.git rcu: fix misplaced mb() in rcu_enter/exit_nohz() In the process of writing up the mechanical proof of correctness for the dynticks/preemptable-RCU interface, I noticed misplaced memory barriers in rcu_enter_nohz() and rcu_exit_nohz(). This patch puts them in the right place and adds a comment. The key thing to keep in mind is that rcu_enter_nohz() is -exiting- the mode that can legally execute RCU read-side critical sections. The memory barrier must be between any potential RCU read-side critical sections and the increment of the per-CPU dynticks_progress_counter, and thus must come -before- this increment. And vice versa for rcu_exit_nohz(). The locking in the scheduler is probably saving us for the moment. Also, switch to smp_mb() - we don't need a barrier for uniprocessor kernels. Signed-off-by: Paul E. McKenney Acked-by: Steven Rostedt Cc: Nick Piggin Cc: Oleg Nesterov Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/include/linux/rcupreempt.h b/include/linux/rcupreempt.h index 01152ed532c8..d038aa6e5ee1 100644 --- a/include/linux/rcupreempt.h +++ b/include/linux/rcupreempt.h @@ -87,15 +87,15 @@ DECLARE_PER_CPU(long, dynticks_progress_counter); static inline void rcu_enter_nohz(void) { + smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */ __get_cpu_var(dynticks_progress_counter)++; WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1); - mb(); } static inline void rcu_exit_nohz(void) { - mb(); __get_cpu_var(dynticks_progress_counter)++; + smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */ WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1)); }