Re: [PATCH] rcu: Fix use __this_cpu_read() warning in preemptible code

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Oct 13, 2022 at 03:39:45PM +0000, Joel Fernandes wrote:
> On Thu, Oct 13, 2022 at 12:41:48PM +0800, Zqiang wrote:
> > BUG: using __this_cpu_read() in preemptible [00000000]
> > code: rcu_torture_fqs/398
> > caller is __this_cpu_preempt_check+0x13/0x20
> > CPU: 3 PID: 398 Comm: rcu_torture_fqs Not tainted 6.0.0-rc1-yoctodev-standard+
> > Call Trace:
> > <TASK>
> > dump_stack_lvl+0x5b/0x86
> > dump_stack+0x10/0x16
> > check_preemption_disabled+0xe5/0xf0
> > __this_cpu_preempt_check+0x13/0x20
> > rcu_force_quiescent_state.part.0+0x1c/0x170
> > rcu_force_quiescent_state+0x1e/0x30
> > rcu_torture_fqs+0xca/0x160
> > ? rcu_torture_boost+0x430/0x430
> > kthread+0x192/0x1d0
> > ? kthread_complete_and_exit+0x30/0x30
> > ret_from_fork+0x22/0x30
> > </TASK>
> > 
> > When enable rcutorture.fqs_duration, the rcu_force_quiescent_state() be
> > invoked in rcu_torture_fqs task context, invoke __this_cpu_read() in
> > preemptible code section will trigger the above calltrace.
> 
> Makes sense, so in theory the GP thread might get migrated after sampling
> rcu_data.mynode but that's Ok since it wont change the behavior of the code
> following it AFAICS.
> 
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Thank you!  I will apply this on the next rebase.

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> > This commit convert __this_cpu_read() to raw_cpu_read() to avoid
> > this warning.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > ---
> >  kernel/rcu/tree.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 96d678c9cfb6..282002e62cf3 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2436,7 +2436,7 @@ void rcu_force_quiescent_state(void)
> >  	struct rcu_node *rnp_old = NULL;
> >  
> >  	/* Funnel through hierarchy to reduce memory contention. */
> > -	rnp = __this_cpu_read(rcu_data.mynode);
> > +	rnp = raw_cpu_read(rcu_data.mynode);
> >  	for (; rnp != NULL; rnp = rnp->parent) {
> >  		ret = (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) ||
> >  		       !raw_spin_trylock(&rnp->fqslock);
> > -- 
> > 2.25.1
> >