linux-kernel - Re: [PATCH] rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass

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Date: Tue, 16 Jan 2024 08:59:17 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Zqiang <qiang.zhang1211@...il.com>, quic_neeraju@...cinc.com,
	joel@...lfernandes.org, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/nocb: Fix WARN_ON_ONCE() in the
 rcu_nocb_bypass_lock()

On Fri, Jan 12, 2024 at 05:09:12PM +0100, Frederic Weisbecker wrote:
> 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@...il.com>
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Queued with Joel's and Frederic's Reviewed-by, thank you all!

							Thanx, Paul