Re: [PATCH] rcu: declare rcu_data variables in the section they are defined in

[Nicolas Iooss <nicolas.iooss_linux@m4x.org>]

On 05/05/2015 04:33 AM, Paul E. McKenney wrote:
> On Sun, May 03, 2015 at 12:27:02PM -0700, Josh Triplett wrote:
>> On Sun, May 03, 2015 at 05:57:53PM +0800, Nicolas Iooss wrote:
>>> Commit 11bbb235c26f ("rcu: Use DEFINE_PER_CPU_SHARED_ALIGNED for
>>> rcu_data") replaced DEFINE_PER_CPU by DEFINE_PER_CPU_SHARED_ALIGNED in
>>> the definition of rcu_sched and rcu_bh without updating
>>> kernel/rcu/tree.h.
>>>
>>> This makes clang report a section mismatch (-Wsection warning) when
>>> building LLVMLinux because the variables are declared in .data..percpu
>>> but defined in .data..percpu..shared_aligned.
>>>
>>> Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
>>
>> Good catch.
>> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> 
> Agreed, good catch!  But don't we also need to worry about
> rcu_preempt_data?

Yes, I missed it because I didn't know that allyesconfig/allmodconfig
does not select CONFIG_PREEMPT (it selects CONFIG_PREEMPT_NONE instead).

> Also, given that tree_trace.c now uses iterators
> rather than direct access via the per-CPU variables, wouldn't the
> following be more appropriate?  (-Very- lightly tested.)

This doesn't work with CONFIG_TREE_RCU_TRACE, because
kernel/rcu/tree_trace.c uses rcu_preempt_state (in v4.1-rc2).  I've
successfully built an allmodconfig+CONFIG_PREEMPT kernel for x86_64 with
the following patch (I have not tested the result).

Anyway, thanks for your quick replies.

Nicolas

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -91,7 +91,7 @@ static const char *tp_##sname##_varname __used
__tracepoint_string = sname##_var

 #define RCU_STATE_INITIALIZER(sname, sabbr, cr) \
 DEFINE_RCU_TPS(sname) \
-DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, sname##_data); \
+static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, sname##_data); \
 struct rcu_state sname##_state = { \
 	.level = { &sname##_state.node[0] }, \
 	.rda = &sname##_data, \
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -519,14 +519,11 @@ extern struct list_head rcu_struct_flavors;
  * RCU implementation internal declarations:
  */
 extern struct rcu_state rcu_sched_state;
-DECLARE_PER_CPU(struct rcu_data, rcu_sched_data);

 extern struct rcu_state rcu_bh_state;
-DECLARE_PER_CPU(struct rcu_data, rcu_bh_data);

 #ifdef CONFIG_PREEMPT_RCU
 extern struct rcu_state rcu_preempt_state;
-DECLARE_PER_CPU(struct rcu_data, rcu_preempt_data);
 #endif /* #ifdef CONFIG_PREEMPT_RCU */

 #ifdef CONFIG_RCU_BOOST