Re: linux-next: build failure after merge of the rcu tree

Paul E. McKenney
2016-01-07T20:52:20Z

On Fri, Jan 08, 2016 at 07:19:32AM +1100, Stephen Rothwell wrote:
> Hi Paul,
>
> On Thu, 7 Jan 2016 10:02:44 -0800 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> >
> > On Thu, Jan 07, 2016 at 07:57:25PM +1100, Stephen Rothwell wrote:
> > > Hi Paul,
> > >
> > > [I found this a few days ago, but I think I forgot to send the email,
> > > sorry.]
> > >
> > > After merging the rcu tree, today's linux-next build (powerpc
> > > allyesconfig) failed like this:
> > >
> > > kernel/rcu/rcuperf.o:(.discard+0x0): multiple definition of `__pcpu_unique_srcu_ctl_srcu_array'
> > > kernel/rcu/rcutorture.o:(.discard+0x0): first defined here
> > >
> > > Caused by commit
> > >
> > > abcd7ec0808e ("rcutorture: Add RCU grace-period performance tests")
> > >
> > > I have reverted that commit for today.
> >
> > Hello, Stephen,
> >
> > Very strange. The "static" keyword does not mean anything here?
> > Easy enough to use different symbols in the two different files,
> > but this situation is not so good for information hiding.
> >
> > Happy to update rcuperf.c to use a different name, but in the
> > immortal words of MSDOS, "Are you sure?" :-)
>
> I have no idea why it happens, but I do get the error above unless I
> revert that commit. So, yes, I am sure :-)
>
> OK, I looked further and
>
> DEFINE_STATIC_SRCU(srcu_ctl);
>
> becomes this (NLs added for clarity):
>
> static __attribute__((section(".discard"), unused)) char __pcpu_scope_srcu_ctl_srcu_array;
> extern __attribute__((section(".discard"), unused)) char __pcpu_unique_srcu_ctl_srcu_array;
> __attribute__((section(".discard"), unused)) char __pcpu_unique_srcu_ctl_srcu_array;
> extern __attribute__((section(".data..percpu" ""))) __typeof__(struct srcu_struct_array) srcu_ctl_srcu_array;
> __attribute__((section(".data..percpu" ""))) __attribute__((weak)) __typeof__(struct srcu_struct_array) srcu_ctl_srcu_array;
> static struct srcu_struct srcu_ctl = {
> .
> .
> };
>
> So, the "static" is not very effective :-(

Oddly enough, this appears to be toolchain dependent. No idea why.

Here is a patch that I will be merging in.

Thanx, Paul

------------------------------------------------------------------------

commit d81f900405de0dc6152692a2088258b8b35d740d
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Thu Jan 7 12:39:10 2016 -0800

Merge with abcd7ec0808e (rcutorture: Add RCU grace-period performance tests)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
index eef82a9460d8..4c8d99aa4f5e 100644
--- a/kernel/rcu/rcuperf.c
+++ b/kernel/rcu/rcuperf.c
@@ -188,8 +188,8 @@ static struct rcu_perf_ops rcu_bh_ops = {
* Definitions for srcu perf testing.
*/

-DEFINE_STATIC_SRCU(srcu_ctl);
-static struct srcu_struct *srcu_ctlp = &srcu_ctl;
+DEFINE_STATIC_SRCU(srcu_ctl_perf);
+static struct srcu_struct *srcu_ctlp = &srcu_ctl_perf;

static int srcu_perf_read_lock(void) __acquires(srcu_ctlp)
{