Re: [PATCH] rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass_lock()

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Jan 10, 2024 at 04:11:28PM +0800, Zqiang wrote:
> For the kernels built with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and
> CONFIG_RCU_LAZY=y, here are the following scenarios that will trigger
> WARN_ON_ONCE() in the rcu_nocb_bypass_lock() and rcu_nocb_wait_contended().
> 
>         CPU2                                               CPU11
> kthread                                                   
> rcu_nocb_cb_kthread                                       ksys_write
> rcu_do_batch                                              vfs_write
> rcu_torture_timer_cb                                      proc_sys_write
> __kmem_cache_free                                         proc_sys_call_handler
> kmemleak_free                                             drop_caches_sysctl_handler
> delete_object_full                                        drop_slab
> __delete_object                                           shrink_slab
> put_object                                                lazy_rcu_shrink_scan
> call_rcu                                                  rcu_nocb_flush_bypass
> __call_rcu_commn                                            rcu_nocb_bypass_lock
>                                                             raw_spin_trylock(&rdp->nocb_bypass_lock) fail
>                                                             atomic_inc(&rdp->nocb_lock_contended);
> rcu_nocb_wait_contended                                     WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
>  WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended))                                          |
>                             |_ _ _ _ _ _ _ _ _ _same rdp and rdp->cpu != 11_ _ _ _ _ _ _ _ _ __|
> 
> This commit therefore use the rcu_nocb_try_flush_bypass() instead of
> rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan(), if the nocb_bypass
> queue is being flushed, the rcu_nocb_try_flush_bypass will return directly.
> 
> Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>

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