Subject: Re: linux-next: build failure after merge of the scsi-mkp tree

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 6 Dec 2017 20:42:22 -0800

On Thu, Dec 07, 2017 at 03:25:21PM +1100, Stephen Rothwell wrote:
> Hi Bart,
> 
> [cc'ing some RCU people ...]
> 
> On Thu, 7 Dec 2017 03:59:30 +0000 Bart Van Assche <[email-masked]> wrote:
> >
> > On Thu, 2017-12-07 at 14:57 +1100, Stephen Rothwell wrote:
> > > After merging the scsi-mkp tree, today's linux-next build (x86_64
> > > allmodconfig) failed like this:
> > > 
> > > ERROR: "init_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
> > > ERROR: "destroy_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
> > > 
> > > Caused by commit
> > > 
> > >   ac90420f17c9 ("scsi: core: Ensure that the SCSI error handler gets woken up")
> > > 
> > > I have used the scsi-mkp tree from next-20171206 for today.  
> > 
> > Does that mean I'm the first one who added RCU code to the SCSI core?
> 
> The only other uses of init_rcu_head() are in drivers/iommu/intel-svm.c
> and kernel/irq/irqdesc.c.  destroy_rcu_head() appears to not be used
> anywhere ...

The key point is that Bart appears to be the first to try using them in
a module, for which exports are needed.  Does the patch below help?

							Thanx, Paul

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

commit cde4691a3a4591e7355295dd62610e3262159002
Author: Paul E. McKenney <[email-masked]>
Date:   Wed Dec 6 20:39:38 2017 -0800

    rcu: Export init_rcu_head() and destroy_rcu_head() to GPL modules
    
    Use of init_rcu_head() and destroy_rcu_head() from modules results in
    the following build-time error:
    
    	ERROR: "init_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
    	ERROR: "destroy_rcu_head" [drivers/scsi/scsi_mod.ko] undefined!
    
    This commit therefore adds EXPORT_SYMBOL_GPL() for each to allow them
    to be used by GPL-licensed kernel modules.
    
    Reported-by: Bart Van Assche <[email-masked]>
    Reported-by: Stephen Rothwell <[email-masked]>
    Signed-off-by: Paul E. McKenney <[email-masked]>

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 8d591d8411fe..4c4d26e9a67b 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -422,11 +422,13 @@ void init_rcu_head(struct rcu_head *head)
 {
 	debug_object_init(head, &rcuhead_debug_descr);
 }
+EXPORT_SYMBOL_GPL(init_rcu_head);
 
 void destroy_rcu_head(struct rcu_head *head)
 {
 	debug_object_free(head, &rcuhead_debug_descr);
 }
+EXPORT_SYMBOL_GPL(destroy_rcu_head);
 
 static bool rcuhead_is_static_object(void *addr)
 {