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Date: Tue, 14 May 2024 17:18:12 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Zqiang <qiang.zhang1211@gmail.com>, neeraj.upadhyay@kernel.org,
	joel@joelfernandes.org, rcu@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu/nocb: Fix using smp_processor_id() in preemptible
 warning
Message-ID: <ZkOAtN9SgTuuVXXT@localhost.localdomain>
References: <20240509074046.15629-1-qiang.zhang1211@gmail.com>
 <d2a9a579-ffcf-4812-a857-2f091f7c65b5@paulmck-laptop>
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Le Tue, May 14, 2024 at 07:54:40AM -0700, Paul E. McKenney a ?crit :
> On Thu, May 09, 2024 at 03:40:46PM +0800, Zqiang wrote:
> > Currently, the this_cpu_ptr(&rcu_data) in rcu_rdp_is_offloaded() is called
> > before the condition "!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())"
> > is checked, and occurs in preemptible task context, this will trigger the
> > following warning.
> > 
> > [ 4.106221][ T18] BUG: using smp_processor_id() in preemptible [00000000] code: rcuop/0/18
> > [ 4.107796][ T18] caller is debug_smp_processor_id (lib/smp_processor_id.c:61)
> > [ 4.108547][ T18] CPU: 0 PID: 18 Comm: rcuop/0 Not tainted 6.9.0-rc2-00079-g4c66bc7cacc0 #1
> > [ 4.109667][ T18] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> > [ 4.111064][ T18] Call Trace:
> > [ 4.111064][ T18]  <TASK>
> > [ 4.111064][ T18] dump_stack_lvl (lib/dump_stack.c:116)
> > [ 4.111064][ T18] dump_stack (lib/dump_stack.c:124)
> > [ 4.111064][ T18] check_preemption_disabled (arch/x86/include/asm/preempt.h:84 (discriminator 15) lib/smp_processor_id.c:53 (discriminator 15))
> > [ 4.111064][ T18] debug_smp_processor_id (lib/smp_processor_id.c:61)
> > [ 4.111064][ T18] rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:27 (discriminator 1))
> > [ 4.111064][ T18] nocb_cb_wait (kernel/rcu/tree_nocb.h:936 (discriminator 2))
> > [ 4.111064][ T18] rcu_nocb_cb_kthread (kernel/rcu/tree_nocb.h:983 (discriminator 1))
> > [ 4.111064][ T18] ? nocb_cb_wait (kernel/rcu/tree_nocb.h:976)
> > [ 4.111064][ T18] kthread (kernel/kthread.c:388)
> > [ 4.111064][ T18] ? kthread (kernel/kthread.c:373 (discriminator 2))
> > [ 4.111064][ T18] ? kthread_complete_and_exit (kernel/kthread.c:341)
> > [ 4.111064][ T18] ret_from_fork (arch/x86/kernel/process.c:153)
> > [ 4.111064][ T18] ? kthread_complete_and_exit (kernel/kthread.c:341)
> > [ 4.111064][ T18] ret_from_fork_asm (arch/x86/entry/entry_64.S:256)
> > [ 4.111064][ T18]  </TASK>
> > 
> > This commit fix this warning by priority check the condition 
> > "!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())" , to
> > ensure whether the this_cpu_ptr(&rcu_data) can be executed in
> > rcu_rdp_is_offloaded().
> > 
> > Fixes: 8feeeba60711 ("rcu/nocb: Use kthread parking instead of ad-hoc implementation")
> > Tested-by: kernel test robot <oliver.sang@intel.com>
> > Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
> 
> Hearing no objections, I have queued this wordsmithed version.  As always,
> please let me know if I have messed anything up.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 5271ad1de0fbcf0bd9caebcf721670c164e5fa9c
> Author: Zqiang <qiang.zhang1211@gmail.com>
> Date:   Thu May 9 15:40:46 2024 +0800
> 
>     rcu/nocb: Don't use smp_processor_id() in preemptible code
>     
>     Currently, rcu_rdp_is_offloaded() invokes this_cpu_ptr(&rcu_data) before
>     the condition "!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())"
>     is checked.  When invoked in preemptible context in preemptible kernels,
>     this will trigger the following warning:
>     
>     [ 4.106221][ T18] BUG: using smp_processor_id() in preemptible [00000000] code: rcuop/0/18
>     [ 4.107796][ T18] caller is debug_smp_processor_id (lib/smp_processor_id.c:61)
>     [ 4.108547][ T18] CPU: 0 PID: 18 Comm: rcuop/0 Not tainted 6.9.0-rc2-00079-g4c66bc7cacc0 #1
>     [ 4.109667][ T18] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>     [ 4.111064][ T18] Call Trace:
>     [ 4.111064][ T18]  <TASK>
>     [ 4.111064][ T18] dump_stack_lvl (lib/dump_stack.c:116)
>     [ 4.111064][ T18] dump_stack (lib/dump_stack.c:124)
>     [ 4.111064][ T18] check_preemption_disabled (arch/x86/include/asm/preempt.h:84 (discriminator 15) lib/smp_processor_id.c:53 (discriminator 15))
>     [ 4.111064][ T18] debug_smp_processor_id (lib/smp_processor_id.c:61)
>     [ 4.111064][ T18] rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:27 (discriminator 1))
>     [ 4.111064][ T18] nocb_cb_wait (kernel/rcu/tree_nocb.h:936 (discriminator 2))
>     [ 4.111064][ T18] rcu_nocb_cb_kthread (kernel/rcu/tree_nocb.h:983 (discriminator 1))
>     [ 4.111064][ T18] ? nocb_cb_wait (kernel/rcu/tree_nocb.h:976)
>     [ 4.111064][ T18] kthread (kernel/kthread.c:388)
>     [ 4.111064][ T18] ? kthread (kernel/kthread.c:373 (discriminator 2))
>     [ 4.111064][ T18] ? kthread_complete_and_exit (kernel/kthread.c:341)
>     [ 4.111064][ T18] ret_from_fork (arch/x86/kernel/process.c:153)
>     [ 4.111064][ T18] ? kthread_complete_and_exit (kernel/kthread.c:341)
>     [ 4.111064][ T18] ret_from_fork_asm (arch/x86/entry/entry_64.S:256)
>     [ 4.111064][ T18]  </TASK>
>     
>     This commit therefore fixes this warning by checking the condition
>     "!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())" before invoking
>     this_cpu_ptr(), thus avoiding preemptible invocations.
>     
>     Fixes: 8feeeba60711 ("rcu/nocb: Use kthread parking instead of ad-hoc implementation")
>     Tested-by: kernel test robot <oliver.sang@intel.com>
>     Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>