[PATCH v3 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT

[Paul E. McKenney]

On Wed, Aug 11, 2021 at 09:13:51PM +0100, Valentin Schneider wrote:
> Running RCU torture with CONFIG_PREEMPT_RT under v5.14-rc4-rt6 triggers:
> 
> [    8.755472] DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt))
> [    8.755482] WARNING: CPU: 1 PID: 137 at kernel/softirq.c:172 __local_bh_disable_ip (kernel/softirq.c:172 (discriminator 31))
> [    8.755500] Modules linked in:
> [    8.755506] CPU: 1 PID: 137 Comm: rcu_torture_rea Not tainted 5.14.0-rc4-rt6 #171
> [    8.755514] Hardware name: ARM Juno development board (r0) (DT)
> [    8.755518] pstate: 60000005 (nZCv daif -PAN -UAO -TCO BTYPE=--)
> [    8.755622] Call trace:
> [    8.755624] __local_bh_disable_ip (kernel/softirq.c:172 (discriminator 31))
> [    8.755633] rcutorture_one_extend (kernel/rcu/rcutorture.c:1453)
> [    8.755640] rcu_torture_one_read (kernel/rcu/rcutorture.c:1601 kernel/rcu/rcutorture.c:1645)
> [    8.755645] rcu_torture_reader (kernel/rcu/rcutorture.c:1737)
> [    8.755650] kthread (kernel/kthread.c:327)
> [    8.755656] ret_from_fork (arch/arm64/kernel/entry.S:783)
> [    8.755663] irq event stamp: 11959
> [    8.755666] hardirqs last enabled at (11959): __rcu_read_unlock (kernel/rcu/tree_plugin.h:695 kernel/rcu/tree_plugin.h:451)
> [    8.755675] hardirqs last disabled at (11958): __rcu_read_unlock (kernel/rcu/tree_plugin.h:661 kernel/rcu/tree_plugin.h:451)
> [    8.755683] softirqs last enabled at (11950): __local_bh_enable_ip (./arch/arm64/include/asm/irqflags.h:85 kernel/softirq.c:261)
> [    8.755692] softirqs last disabled at (11942): rcutorture_one_extend (./include/linux/bottom_half.h:19 kernel/rcu/rcutorture.c:1441)
> 
> Per this warning, softirqs cannot be disabled in a non-preemptible region
> under CONFIG_PREEMPT_RT. Adjust RCU torture accordingly.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider at arm.com>

Looks sensible for -rt, but of course does not apply on mainline or -rcu.

Acked-by: Paul E. McKenney <paulmck at kernel.org>

> ---
>  kernel/rcu/rcutorture.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index eecd1caef71d..4f3db1d3170d 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1548,6 +1548,8 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
>  	 * them on non-RT.
>  	 */
>  	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		/* Can't disable bh in atomic context under PREEMPT_RT */
> +		mask &= ~(RCUTORTURE_RDR_ATOM_BH | RCUTORTURE_RDR_ATOM_RBH);
>  		/*
>  		 * Can't release the outermost rcu lock in an irq disabled
>  		 * section without preemption also being disabled, if irqs
> -- 
> 2.25.1
>