+The kernel needs to access user-space memory, for example, to access +data referenced by system-call parameters. +The get_user() macro does this job. + +
+However, user-space memory might well be paged out, which means +that get_user() might well page-fault and thus block while +waiting for the resulting I/O to complete. +It would be a very bad thing for the compiler to reorder +a get_user() invocation into an RCU read-side critical +section. +For example, suppose that the source code looked like this: + +
++ ++ 1 rcu_read_lock(); + 2 p = rcu_dereference(gp); + 3 v = p->value; + 4 rcu_read_unlock(); + 5 get_user(user_v, user_p); + 6 do_something_with(v, user_v); ++
+The compiler must not be permitted to transform this source code into +the following: + +
++ ++ 1 rcu_read_lock(); + 2 p = rcu_dereference(gp); + 3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!! + 4 v = p->value; + 5 rcu_read_unlock(); + 6 do_something_with(v, user_v); ++
+If the compiler did make this transformation in a +CONFIG_PREEMPT=n kernel build, and if get_user() did +page fault, the result would be a quiescent state in the middle +of an RCU read-side critical section. +This misplaced quiescent state could result in line 4 being +a use-after-free access, which could be bad for your kernel's +actuarial statistics. +Similar examples can be constructed with the call to get_user() +preceding the rcu_read_lock(). + +
+Unfortunately, get_user() doesn't have any particular +ordering properties, and in some architectures the underlying asm +isn't even marked volatile. +And even if it was marked volatile, the above access to +p->value is not volatile, so the compiler would not have any +reason to keep those two accesses in order. + +
+Therefore, the Linux-kernel definitions of rcu_read_lock() +and rcu_read_unlock() must act as compiler barriers, +at least for outermost instances of rcu_read_lock() and +rcu_read_unlock() within a nested set of RCU read-side critical +sections. +
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index acb225023ed1..3f1b5041de9b 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -288,7 +288,6 @@ void rcu_note_context_switch(bool preempt) struct rcu_data *rdp = this_cpu_ptr(&rcu_data); struct rcu_node *rnp; - barrier(); /* Avoid RCU read-side critical sections leaking down. */ trace_rcu_utilization(TPS("Start context switch")); lockdep_assert_irqs_disabled(); WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0); @@ -340,7 +339,6 @@ void rcu_note_context_switch(bool preempt) if (rdp->exp_deferred_qs) rcu_report_exp_rdp(rdp); trace_rcu_utilization(TPS("End context switch")); - barrier(); /* Avoid RCU read-side critical sections leaking up. */ } EXPORT_SYMBOL_GPL(rcu_note_context_switch); @@ -828,11 +826,6 @@ static void rcu_qs(void) * dyntick-idle quiescent state visible to other CPUs, which will in * some cases serve for expedited as well as normal grace periods. * Either way, register a lightweight quiescent state. - * - * The barrier() calls are redundant in the common case when this is - * called externally, but just in case this is called from within this - * file. - * */ void rcu_all_qs(void) { @@ -847,14 +840,12 @@ void rcu_all_qs(void) return; } this_cpu_write(rcu_data.rcu_urgent_qs, false); - barrier(); /* Avoid RCU read-side critical sections leaking down. */ if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) { local_irq_save(flags); rcu_momentary_dyntick_idle(); local_irq_restore(flags); } rcu_qs(); - barrier(); /* Avoid RCU read-side critical sections leaking up. */ preempt_enable(); } EXPORT_SYMBOL_GPL(rcu_all_qs); @@ -864,7 +855,6 @@ EXPORT_SYMBOL_GPL(rcu_all_qs); */ void rcu_note_context_switch(bool preempt) { - barrier(); /* Avoid RCU read-side critical sections leaking down. */ trace_rcu_utilization(TPS("Start context switch")); rcu_qs(); /* Load rcu_urgent_qs before other flags. */ @@ -877,7 +867,6 @@ void rcu_note_context_switch(bool preempt) rcu_tasks_qs(current); out: trace_rcu_utilization(TPS("End context switch")); - barrier(); /* Avoid RCU read-side critical sections leaking up. */ } EXPORT_SYMBOL_GPL(rcu_note_context_switch);