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

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Message-ID: <YjNll+Iv++LORS0n@ip-172-31-19-208.ap-northeast-1.compute.internal>
Date:   Thu, 17 Mar 2022 16:45:11 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 09:20:33AM -0700, Paul E. McKenney wrote:
> 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?
>

TASKS_RUDE_RCU=n
TASKS_TRACE_RCU=n
TASKS_RCU_GENERIC=y
TASKS_RCU=y

> > > 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?
>

Right.

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

No. did not help.

I think I found reason...
with PREEMPTION=y,

in kernel/rcu/Kconfig:
config TASKS_RCU
        def_bool PREEMPTION
        help
          This option enables a task-based RCU implementation that uses
          only voluntary context switch (not preemption!), idle, and 
          user-mode execution as quiescent states.  Not for manual selection.

in kernel/rcu/Kconfig:
config TASKS_RCU_GENERIC
        def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
        select SRCU
        help
          This option enables generic infrastructure code supporting
          task-based RCU implementations.  Not for manual selection.


> > > > 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.  ;-)

Sorry to make you guess. Maybe too late, but added config as attachment ;)

> 
> > 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?
>

No, this is selected. TASKS_RCU_GENERIC=y. because of PREEMPTION=y.

Thanks!

> 							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

-- 
Thank you, You are awesome!
Hyeonggon :-)

View attachment ".config" of type "text/plain" (32165 bytes)