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Date:   Wed, 22 Mar 2023 22:07:37 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@intel.com>
Cc:     "frederic@kernel.org" <frederic@kernel.org>,
        "joel@joelfernandes.org" <joel@joelfernandes.org>,
        "rcu@vger.kernel.org" <rcu@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "qiang.zhang1211@gmail.com" <qiang.zhang1211@gmail.com>
Subject: Re: [PATCH] srcu: Fix flush sup work warning in cleanup_srcu_struct()
Message-ID: <6824439a-20c3-4635-a34a-44ef57769cce@paulmck-laptop>
Reply-To: paulmck@kernel.org
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On Thu, Mar 23, 2023 at 03:39:12AM +0000, Zhang, Qiang1 wrote:
> > >On Wed, Mar 22, 2023 at 10:08:54PM +0000, Zhang, Qiang1 wrote:
> > > > > > insmod rcutorture.ko
> > > > > > rmmod rcutorture.ko
> > > > > > 
> > > > > > [  209.437327] WARNING: CPU: 0 PID: 508 at kernel/workqueue.c:3167 
> > > > > > __flush_work+0x50a/0x540 [  209.437346] Modules linked in: 
> > > > > > rcutorture(-) torture [last unloaded: rcutorture] [  209.437382] 
> > > > > > CPU: 0 PID: 508 Comm: rmmod Tainted: G  W  6.3.0-rc1-yocto-standard+ 
> > > > > > [  209.437406] RIP: 0010:__flush_work+0x50a/0x540 .....
> > > > > > [  209.437758]  flush_delayed_work+0x36/0x90 [  209.437776]  
> > > > > > cleanup_srcu_struct+0x68/0x2e0 [  209.437817]  
> > > > > > srcu_module_notify+0x71/0x140 [  209.437854]  
> > > > > > blocking_notifier_call_chain+0x9d/0xd0
> > > > > > [  209.437880]  __x64_sys_delete_module+0x223/0x2e0
> > > > > > [  209.438046]  do_syscall_64+0x43/0x90 [  209.438062]  
> > > > > > entry_SYSCALL_64_after_hwframe+0x72/0xdc
> > > > > > 
> > > > > > For srcu objects defined with DEFINE_SRCU() or DEFINE_STATIC_SRCU(), 
> > > > > > when compiling and loading as modules, the srcu_module_coming() is 
> > > > > > invoked, allocate memory for srcu structure's->sda and initialize 
> > > > > > sda structure, due to not fully initialize srcu structure's->sup, so 
> > > > > > at this time the sup structure's->delaywork.func is null, if not 
> > > > > > invoke init_srcu_struct_fields() before unloading modules, in 
> > > > > > srcu_module_going() the __flush_work() find
> > > > > > work->func is empty, will raise the warning above.
> > > > > > 
> > > > > > This commit add init_srcu_struct_fields() to initialize srcu 
> > > > > > structure's->sup, in srcu_module_coming().
> > > > > > 
> > > > > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > > > >
> > > > > >Good catch, and thank you for testing the in-module case!
> > > > > >
> > > > > >One question below...
> > > > > >
> > > > > >							Thanx, Paul
> > > > > >
> > > > > > ---
> > > > > >  kernel/rcu/srcutree.c | 11 ++++++++---
> > > > > >  1 file changed, 8 insertions(+), 3 deletions(-)
> > > > > > 
> > > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 
> > > > > > 1fb078abbdc9..42d8720e016c 100644
> > > > > > --- a/kernel/rcu/srcutree.c
> > > > > > +++ b/kernel/rcu/srcutree.c
> > > > > > @@ -1921,7 +1921,8 @@ static int srcu_module_coming(struct module *mod)
> > > > > >  		ssp->sda = alloc_percpu(struct srcu_data);
> > > > > >  		if (WARN_ON_ONCE(!ssp->sda))
> > > > > >  			return -ENOMEM;
> > > > > > -		init_srcu_struct_data(ssp);
> > > > > > +		if (WARN_ON_ONCE(init_srcu_struct_fields(ssp, true)))
> > > > > > +			return -ENOMEM;
> > > > > >
> > > > > >Wouldn't it be better to simply delete the init_srcu_struct_data()?
> > > > > >
> > > > > >Then the first call to check_init_srcu_struct() would take care of 
> > > > > >the initialization, just as for the non-module case.  Or am I missing 
> > > > > >something subtle?
> > > > > 
> > > > > Hi Paul
> > > > > 
> > > > > Maybe the check_init_srcu_struct() is always not invoked, for example,
> > > > > In rcutorture.c,   here is such a definition DEFINE_STATIC_SRCU(srcu_ctl),
> > > > > but we use torture_type=rcu to test,  there will not be any interface 
> > > > > calling
> > > > > check_init_srcu_struct() to initialize srcu_ctl and set  
> > > > > structure's->delaywork.func is process_srcu().
> > > > > when we unload the rcutorture module, invoke cleanup_srcu_struct() to 
> > > > > flush sup structure's->delaywork.func, due to the func pointer is not 
> > > > > initialize, it's null, will trigger warning.
> > > > > 
> > > > > About kernel/workqueue.c:3167
> > > > > 
> > > > > __flush_work
> > > > >      if (WARN_ON(!work->func))   <---------trigger waning
> > > > > 	return false;
> > > > > 
> > > > > 
> > > > > and  in  init_srcu_struct_fields(ssp, true), wil set 
> > > > > srcu_sup->sda_is_static is true and set srcu_sup-> srcu_gp_seq_needed 
> > > > > is zero,  after that when we call
> > > > >  check_init_srcu_struct() again, it not be initialized again.
> > > > >
> > > > >
> > > > >Good point!  In the non-module statically allocated case there is never a call to cleanup_srcu_struct().
> > > > >
> > > > >So suppose the code in srcu_module_coming() only did the alloc_percpu(), and then the
> > > > >code in srcu_module_going() only did the the matching
> > > > >free_percpu() instead of the current cleanup_srcu_struct()?
> > > > 
> > > > But in modules, for srcu objects defined with DEFINE_SRCU() or DEFINE_STATIC_SRCU(),
> > > > when a module is unloaded, we usually don't call cleanup_srcu_struct() in the module
> > > > unload function.
> > > > If in srcu_module_going() only do free_percpu(), the srcu_sup->node memory maybe
> > > > can not free and also lost the opportunity to refresh the running work.
> > > >
> > > >
> > > >But in the module case, isn't the srcu_sup->node also statically
> > > >allocated via the "static struct srcu_usage" declaration?
> > > 
> > > static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
> > > {
> > > 	sp->srcu_sup->node = kcalloc(rcu_num_nodes, sizeof(*ssp->srcu_sup->node), gfp_flags);
> > > 	...
> > > }
> > > 
> > > Regardless of whether the srcu object is declared in the module or not, sup->node is dynamically allocated.
> > > right?
> > >
> > >You are absolutely right, thank you!
> > >
> > >There are a couple of ways to resolve this.  One is to simply add
> > >a check_init_srcu_struct() before the call to cleanup_srcu_struct()
> > >from srcu_module_going(), as shown below.  This seems a bit silly,
> > >potentially initializing fields for no good reason.
> > >
> > >Another way is to make cleanup_srcu_struct() do the same check
> > >that check_init_srcu_struct() does:
> > >
> > >	rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq_needed))
> > >
> > >If the value is non-zero, then cleanup_srcu_struct() should skip
> > >consistency checks that complain about things that cannot happen if
> > >there never was an RCU grace period.  Maybe something as shown after
> > >the second line of dashes.
> > >
> > >Thoughts?
> > >
> > >							Thanx, Paul
> > >
> > >------------------------------------------------------------------------
> > >
> > >
> > >/* Initialize any global-scope srcu_struct structures used by this module. */
> > >static int srcu_module_coming(struct module *mod)
> > >{
> > >	int i;
> > >	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
> > >	struct srcu_struct *ssp;
> > >
> > >	for (i = 0; i < mod->num_srcu_structs; i++) {
> > >		ssp = *(sspp++);
> > >		ssp->sda = alloc_percpu(struct srcu_data);
> > >		if (WARN_ON_ONCE(!ssp->sda))
> > >			return -ENOMEM;
> > >		init_srcu_struct_data(ssp);
> > >	}
> > >	return 0;
> > >}
> > >
> > >/* Clean up any global-scope srcu_struct structures used by this module. */
> > >static void srcu_module_going(struct module *mod)
> > >{
> > >	int i;
> > >	struct srcu_struct *ssp;
> > >	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
> > >
> > >	for (i = 0; i < mod->num_srcu_structs; i++) {
> > >		ssp = *(sspp++);
> > >		check_init_srcu_struct(ssp);
> > >		cleanup_srcu_struct(ssp);
> > >	}
> > >}
> > >
> > >------------------------------------------------------------------------
> > >
> > >void cleanup_srcu_struct(struct srcu_struct *ssp)
> > >{
> > >	int cpu;
> > >	struct srcu_usage *sup = ssp->srcu_sup;
> > >	bool wasused = !rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq_needed));
> > >
> > >	if (WARN_ON(wasused && !srcu_get_delay(ssp)))
> > >		return; /* Just leak it! */
> > >	if (WARN_ON(srcu_readers_active(ssp)))
> > >		return; /* Just leak it! */
> > >	flush_delayed_work(&sup->work);
> > >	if (wasused) {
> > 
> >   If   wasused=false  It not need to invoke   flush_delayed_work(&sup->work);
> >   this trigger WARN_ON(!work->func)) .
> >
> >Again, good catch!  I will pull that flush_delayed_work() into this
> >"if" statement.
> >
> > >		for_each_possible_cpu(cpu) {
> > >			struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
> > >
> > >			del_timer_sync(&sdp->delay_work);
> > >			flush_work(&sdp->work);
> > >			if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
> > >				return; /* Forgot srcu_barrier(), so just leak it! */
> > >		}
> > >	}
> > >	if (WARN_ON(wasused && rcu_seq_state(READ_ONCE(sup->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
> > >	    WARN_ON(wasused && rcu_seq_current(&sup->srcu_gp_seq) != sup->srcu_gp_seq_needed) ||
> > >	    WARN_ON(srcu_readers_active(ssp))) {
> > >		pr_info("%s: Active srcu_struct %p read state: %d gp state: %lu/%lu\n",
> > >			__func__, ssp, rcu_seq_state(READ_ONCE(sup->srcu_gp_seq)),
> > >			rcu_seq_current(&sup->srcu_gp_seq), sup->srcu_gp_seq_needed);
> > >		return; /* Caller forgot to stop doing call_srcu()? */
> > >	}
> > >	kfree(sup->node);
> > >	sup->node = NULL;
> > >	sup->srcu_size_state = SRCU_SIZE_SMALL;
> > >	if (!sup->sda_is_static) {
> > >		free_percpu(ssp->sda);
> > >		ssp->sda = NULL;
> > >		kfree(sup);
> > >		ssp->srcu_sup = NULL;
> > >	}
> > >}
> > 
> > 
> > If we have not invoke check_init_srcu_struct() ,  that means call_srcu(),  synchronize_srcu(), srcu_barrier(), start_poll_synchronize_srcu() are also not invoke, so Is there no need to check
> > srcu_readers_active()?
> >
> >The module might have had lots of SRCU readers, but no updaters, and
> >a bug in that module might mean that that readers are still active.
> >For example, the module might have passed the srcu_struct structure
> >to some function in the main kernel, and then forgotten to tell that
> >function to stop doing srcu_read_lock() and srcu_read_unlock() on it.
> >
> >Or the module might have created a kthread that did SRCU readers, and
> >then have forgotten to stop that kthread.
> >
> >
> >fully understand.
> >
> >
> >Please see below for an untested patch.
> >
> >I will test the following modification.
> >
> >Thanks
> >Zqiang
> >
> >
> >And yet again, thoughts?  ;-)
> 
> Maybe add the following modification,  otherwise, in cleanup_srcu_struct()
> Kfree(sup) will release ssp->srcu_sup which is statically allocated.
> 
> @@ -1926,6 +1926,7 @@ static int srcu_module_coming(struct module *mod)
>                 if (WARN_ON_ONCE(!ssp->sda))
>                         return -ENOMEM;
>                 init_srcu_struct_data(ssp);
> +               ssp->srcu_sup->sda_is_static = true;
>         }

Good catch and good point!

But the underlying problem is that I am still making things too complex.

How about the following?  The idea is to skip the cleanup_srcu_struct()
unless there was a call to check_init_srcu_struct(), and to free the
per-CPU data either way.

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 1fb078abbdc9..06f8ed1ce272 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1921,7 +1921,6 @@ static int srcu_module_coming(struct module *mod)
 		ssp->sda = alloc_percpu(struct srcu_data);
 		if (WARN_ON_ONCE(!ssp->sda))
 			return -ENOMEM;
-		init_srcu_struct_data(ssp);
 	}
 	return 0;
 }
@@ -1930,10 +1929,17 @@ static int srcu_module_coming(struct module *mod)
 static void srcu_module_going(struct module *mod)
 {
 	int i;
+	struct srcu_data __percpu *sda;
+	struct srcu_struct *ssp;
 	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
 
-	for (i = 0; i < mod->num_srcu_structs; i++)
-		cleanup_srcu_struct(*(sspp++));
+	for (i = 0; i < mod->num_srcu_structs; i++) {
+		ssp = *(sspp++);
+		sda = ssp->sda;
+		if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq_needed)))
+			cleanup_srcu_struct(ssp);
+		free_percpu(sda);
+	}
 }
 
 /* Handle one module, either coming or going. */