linux-kernel - Re: RCU: undefined reference to irq_work

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Date:   Thu, 17 Mar 2022 09:20:33 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     rcu@...r.kernel.org, Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org
Subject: Re: RCU: undefined reference to irq_work_queue

On Thu, Mar 17, 2022 at 03:24:42PM +0000, Hyeonggon Yoo wrote:
> On Thu, Mar 17, 2022 at 07:00:00AM -0700, Paul E. McKenney wrote:
> > On Thu, Mar 17, 2022 at 11:32:53AM +0000, Hyeonggon Yoo wrote:
> > > Hello RCU folks,
> > > 
> > > I like to use minimal configuration for kernel development.
> > > when building with tinyconfig + CONFIG_PREEMPT=y on arm64:
> > > 
> > > ld: kernel/rcu/update.o: in function `call_rcu_tasks':
> > > update.c:(.text+0xb2c): undefined reference to `irq_work_queue'
> > > update.c:(.text+0xb2c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `irq_work_queue'
> > > make: *** [Makefile:1155: vmlinux] Error 1
> > > 
> > > It seems RCU calls irq_work_queue() without checking if CONFIG_IRQ_WORK is enabled.
> > 
> > Indeed it does!
> > 
> > And kernel/rcu/Kconfig shows why:
> > 
> > config TASKS_TRACE_RCU
> > 	def_bool 0
> > 	select IRQ_WORK
> > 	help
> > 	  This option enables a task-based RCU implementation that uses
> > 	  explicit rcu_read_lock_trace() read-side markers, and allows
> > 	  these readers to appear in the idle loop as well as on the CPU
> > 	  hotplug code paths.  It can force IPIs on online CPUs, including
> > 	  idle ones, so use with caution.
> > 
> > So the solution is to further minimize your configuration so as to
> > deselect TASKS_TRACE_RCU.
> 
> They are already not selected.

Good, thank you.

How about TASKS_RUDE_RCU, TASKS_TRACE_RCU, and TASKS_RCU_GENERIC?

> > This means making sure that both BPF and
> > the various RCU torture tests are all deselected.
> 
> I wanted to say call_rcu_tasks() can be referenced even when IRQ_WORK is not
> selected, making it fail to build.

I am guessing because TASKS_RCU_GENERIC is selected?

If so, does the patch at the end of this email help?

> > > ld: kernel/rcu/update.o: in function `call_rcu_tasks':
> > > update.c:(.text+0xb2c): undefined reference to `irq_work_queue'
> > > update.c:(.text+0xb2c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `irq_work_queue'
> > > make: *** [Makefile:1155: vmlinux] Error 1
> 
> Isn't it better to fix this build failure?

But of course!  However, first I need to know exactly what is causing your
build failure.  I cannot see your .config file, so I am having to guess.

Don't get me wrong, I do have a lot of practice guessing, but it is still
just guessing.  ;-)

> It fails to build when both TASKS_TRACE_RCU and IRQ_WORK are not selected
> and PREEMPT is selected.
> 
>   │ Symbol: TASKS_TRACE_RCU [=n]                                            │
>   │ Type  : bool                                                            │
>   │ Defined at kernel/rcu/Kconfig:96                                        │
>   │ Selects: IRQ_WORK [=n]                                                  │
>   │ Selected by [n]:                                                        │
>   │   - BPF_SYSCALL [=n]                                                    │
>   │   - RCU_SCALE_TEST [=n] && DEBUG_KERNEL [=y]                            │
>   │   - RCU_TORTURE_TEST [=n] && DEBUG_KERNEL [=y]                          │
>   │   - RCU_REF_SCALE_TEST [=n] && DEBUG_KERNEL [=y]
> 
> Thanks!
> 
> > 
> > Or turn on IRQ_WORK, for example, if you need to use BPF.

Or do you already have TASKS_RCU_GENERIC deselected?

							Thanx, Paul

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

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index bf8e341e75b4..f559870fbf8b 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -86,6 +86,7 @@ config TASKS_RCU
 
 config TASKS_RUDE_RCU
 	def_bool 0
+	select IRQ_WORK
 	help
 	  This option enables a task-based RCU implementation that uses
 	  only context switch (including preemption) and user-mode