Message-ID: <20221116195719.GV4001@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 16 Nov 2022 11:57:19 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     "Zhang, Qiang1" <qiang1.zhang@...el.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] rcu: Dump memory object info if callback function is
 invalid

On Wed, Nov 16, 2022 at 10:43:57PM +0800, Leizhen (ThunderTown) wrote:
> On 2022/11/15 0:06, Paul E. McKenney wrote:
> > On Mon, Nov 14, 2022 at 03:18:10PM +0800, Leizhen (ThunderTown) wrote:
> >> On 2022/11/12 14:08, Paul E. McKenney wrote:
> >>> On Sat, Nov 12, 2022 at 10:32:32AM +0800, Leizhen (ThunderTown) wrote:
> >>>> On 2022/11/12 2:42, Paul E. McKenney wrote:
> >>>>> On Fri, Nov 11, 2022 at 01:05:56PM +0000, Zhang, Qiang1 wrote:
> >>>>>> On 2022/11/11 19:54, Zhang, Qiang1 wrote:
> >>>>>>>> When a structure containing an RCU callback rhp is (incorrectly) 
> >>>>>>>> freed and reallocated after rhp is passed to call_rcu(), it is not 
> >>>>>>>> unusual for
> >>>>>>>> rhp->func to be set to NULL. This defeats the debugging prints used 
> >>>>>>>> rhp->by
> >>>>>>>> __call_rcu_common() in kernels built with 
> >>>>>>>> CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, which expect to identify the 
> >>>>>>>> offending code using the identity of this function.
> >>>>>>>>
> >>>>>>>> And in kernels build without CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, things 
> >>>>>>>> are even worse, as can be seen from this splat:
> >>>>>>>>
> >>>>>>>> Unable to handle kernel NULL pointer dereference at virtual address 0 
> >>>>>>>> ... ...
> >>>>>>>> PC is at 0x0
> >>>>>>>> LR is at rcu_do_batch+0x1c0/0x3b8
> >>>>>>>> ... ...
> >>>>>>>> (rcu_do_batch) from (rcu_core+0x1d4/0x284)
> >>>>>>>> (rcu_core) from (__do_softirq+0x24c/0x344)
> >>>>>>>> (__do_softirq) from (__irq_exit_rcu+0x64/0x108)
> >>>>>>>> (__irq_exit_rcu) from (irq_exit+0x8/0x10)
> >>>>>>>> (irq_exit) from (__handle_domain_irq+0x74/0x9c)
> >>>>>>>> (__handle_domain_irq) from (gic_handle_irq+0x8c/0x98)
> >>>>>>>> (gic_handle_irq) from (__irq_svc+0x5c/0x94)
> >>>>>>>> (__irq_svc) from (arch_cpu_idle+0x20/0x3c)
> >>>>>>>> (arch_cpu_idle) from (default_idle_call+0x4c/0x78)
> >>>>>>>> (default_idle_call) from (do_idle+0xf8/0x150)
> >>>>>>>> (do_idle) from (cpu_startup_entry+0x18/0x20)
> >>>>>>>> (cpu_startup_entry) from (0xc01530)
> >>>>>>>>
> >>>>>>>> This commit therefore adds calls to mem_dump_obj(rhp) to output some 
> >>>>>>>> information, for example:
> >>>>>>>>
> >>>>>>>>  slab kmalloc-256 start ffff410c45019900 pointer offset 0 size 256
> >>>>>>>>
> >>>>>>>> This provides the rough size of the memory block and the offset of 
> >>>>>>>> the rcu_head structure, which as least provides at least a few clues 
> >>>>>>>> to help locate the problem. If the problem is reproducible, 
> >>>>>>>> additional slab debugging can be enabled, for example, 
> >>>>>>>> CONFIG_DEBUG_SLAB=y, which can provide significantly more information.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> >>>>>>>> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >>>>>>>> ---
> >>>>>>>> kernel/rcu/rcu.h      | 7 +++++++
> >>>>>>>> kernel/rcu/srcutiny.c | 1 +
> >>>>>>>> kernel/rcu/srcutree.c | 1 +
> >>>>>>>> kernel/rcu/tasks.h    | 1 +
> >>>>>>>> kernel/rcu/tiny.c     | 1 +
> >>>>>>>> kernel/rcu/tree.c     | 1 +
> >>>>>>>> 6 files changed, 12 insertions(+)
> >>>>>>>>
> >>>>>>>> v1 --> v2:
> >>>>>>>> 1. Remove condition "(unsigned long)rhp->func & 0x3", it have problems on x86.
> >>>>>>>> 2. Paul E. McKenney helped me update the commit message, thanks.
> >>>>>>>>
> >>>>>>>
> >>>>>>> Hi, Zhen Lei
> >>>>>>>
> >>>>>>> Maybe the following scenarios should be considered:
> >>>>>>>
> >>>>>>>                 CPU 0
> >>>>>>> tasks context
> >>>>>>>    spin_lock(&vmap_area_lock)
> >>>>>>>           Interrupt 
> >>>>>>> 	 RCU softirq
> >>>>>>> 	      rcu_do_batch
> >>>>>>> 		mem_dump_obj
> >>>>>>>                                   vmalloc_dump_obj
> >>>>>>>                                        spin_lock(&vmap_area_lock)   <--  deadlock     
> >>>>>>
> >>>>>>> Right, thanks. I just saw the robot's report. So this patch should be dropped.
> >>>>>>> I'll try to add an helper in mm, where I can check whether the lock has been held, and dump the content of memory object.
> >>>>>>
> >>>>>> This is a workaround, or maybe try a modification like the following, 
> >>>>>> of course, need to ask Paul's opinion.
> >>>>>
> >>>>> Another approach is to schedule a workqueue handler to do the
> >>>>> mem_dump_obj().  This would allow mem_dump_obj() to run in a clean
> >>>>> environment.
> >>>>
> >>>> It's about to panic, so no chance to schedule.
> >>>
> >>> It won't panic if you drop the callback on the floor.
> >>>
> >>> Though to your point, the ->next pointer is likely also trashed.  So you
> >>> could just drop the remainder of the callback list on the floor.  That
> >>> might provide a good (though not perfect) chance of getting decent output.
> >>
> >> OK, I think I understand what you mean.
> >> if (!f)
> >> 	schedule_work(&work);
> >> else
> >> 	f(rhp)
> > 
> > Yes, except that the "schedule_work()" also needs to be accompanied
> > by something that refuses to execute the rest of those callbacks.
> > This needs to break out of the loop (or return) and to adjust counts,
> > among other things.  This might be as easy as setting count to the
> > negative of the length of the "rcl" list, but does need some attention
> > to the code following the callback-invocation loop.
> 
> Yes, doing so would cause other problems. As you mentioned, the ->next
> pointer is likely also trashed. Some nodes may need to be executed in
> sequence. For such a weak debug function, it's not worth the risk, or
> overly complicated thinking.

Do we have similar deadlock issues with the calls to mem_dump_obj() in
the call_rcu() code path?  These are somewhat less concerning because
they are invoked under a Kconfig option that is (as far as I know)
rarely set, but still...

							Thanx, Paul

> >>>>> This would allow vmalloc_dump_obj() to be called unconditionally.
> >>>>>
> >>>>> Other thoughts?
> >>>>
> >>>> locked = spin_is_locked(&vmap_area_lock);
> >>>> if (!locked)
> >>>>     spin_lock(&vmap_area_lock)
> >>>>
> >>>> Careful analysis is required, which may cause other problems.
> >>>>
> >>>> Or in new function:
> >>>> if (locked)
> >>>>     return;
> >>>> spin_lock(&vmap_area_lock);
> >>>>
> >>>> If there is a chance to dump the data, dump the data. If there is no
> >>>> chance to dump the data, do not dump the data. This is the fate of
> >>>> debugging information.
> >>>
> >>> My concern is that there will be increasing numbers of special cases
> >>> over time.
> 
> The memory modules are mature and stable, so your concerns may not be true.
> 
> >>
> >> OK, I got it.
> >>
> >>>
> >>> 							Thanx, Paul
> >>>
> >>>>>> diff --git a/mm/util.c b/mm/util.c
> >>>>>> index 12984e76767e..86da0739fe5d 100644
> >>>>>> --- a/mm/util.c
> >>>>>> +++ b/mm/util.c
> >>>>>> @@ -1119,14 +1119,18 @@ void mem_dump_obj(void *object)
> >>>>>>  {
> >>>>>>         const char *type;
> >>>>>>
> >>>>>> +       if (is_vmalloc_addr(object)) {
> >>>>>> +               if (in_task() && vmalloc_dump_obj(object))
> >>>>>> +                       return;
> >>>>>> +               type = "vmalloc memory";
> >>>>>> +               goto end;
> >>>>>> +       }
> >>>>>> +
> >>>>>>         if (kmem_valid_obj(object)) {
> >>>>>>                 kmem_dump_obj(object);
> >>>>>>                 return;
> >>>>>>         }
> >>>>>>
> >>>>>> -       if (vmalloc_dump_obj(object))
> >>>>>> -               return;
> >>>>>> -
> >>>>>>         if (virt_addr_valid(object))
> >>>>>>                 type = "non-slab/vmalloc memory";
> >>>>>>         else if (object == NULL)
> >>>>>> @@ -1135,7 +1139,7 @@ void mem_dump_obj(void *object)
> >>>>>>                 type = "zero-size pointer";
> >>>>>>         else
> >>>>>>                 type = "non-paged memory";
> >>>>>> -
> >>>>>> +end:
> >>>>>>         pr_cont(" %s\n", type);
> >>>>>>  }
> >>>>>>  EXPORT_SYMBOL_GPL(mem_dump_obj);
> >>>>>>
> >>>>>> Thanks
> >>>>>> Zqiang
> >>>>>>
> >>>>>>
> >>>>>>>
> >>>>>>> Thanks
> >>>>>>> Zqiang
> >>>>>>>
> >>>>>>>
> >>>>>>>> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 
> >>>>>>>> 65704cbc9df7b3d..32ab45fabf8eebf 100644
> >>>>>>>> --- a/kernel/rcu/rcu.h
> >>>>>>>> +++ b/kernel/rcu/rcu.h
> >>>>>>>> @@ -10,6 +10,7 @@
> >>>>>>>> #ifndef __LINUX_RCU_H
> >>>>>>>> #define __LINUX_RCU_H
> >>>>>>>>
> >>>>>>>> +#include <linux/mm.h>
> >>>>>>>> #include <trace/events/rcu.h>
> >>>>>>>>
> >>>>>>>> /*
> >>>>>>>> @@ -211,6 +212,12 @@ static inline void debug_rcu_head_unqueue(struct 
> >>>>>>>> rcu_head *head) }
> >>>>>>>> #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
> >>>>>>>>
> >>>>>>>> +static inline void debug_rcu_head_callback(struct rcu_head *rhp) {
> >>>>>>>> +	if (unlikely(!rhp->func))
> >>>>>>>> +		mem_dump_obj(rhp);
> >>>>>>>> +}
> >>>>>>>> +
> >>>>>>>> extern int rcu_cpu_stall_suppress_at_boot;
> >>>>>>>>
> >>>>>>>> static inline bool rcu_stall_is_suppressed_at_boot(void)
> >>>>>>>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c index 
> >>>>>>>> 33adafdad261389..5e7f336baa06ae0 100644
> >>>>>>>> --- a/kernel/rcu/srcutiny.c
> >>>>>>>> +++ b/kernel/rcu/srcutiny.c
> >>>>>>>> @@ -138,6 +138,7 @@ void srcu_drive_gp(struct work_struct *wp)
> >>>>>>>> 	while (lh) {
> >>>>>>>> 		rhp = lh;
> >>>>>>>> 		lh = lh->next;
> >>>>>>>> +		debug_rcu_head_callback(rhp);
> >>>>>>>> 		local_bh_disable();
> >>>>>>>> 		rhp->func(rhp);
> >>>>>>>> 		local_bh_enable();
> >>>>>>>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 
> >>>>>>>> ca4b5dcec675bac..294972e66b31863 100644
> >>>>>>>> --- a/kernel/rcu/srcutree.c
> >>>>>>>> +++ b/kernel/rcu/srcutree.c
> >>>>>>>> @@ -1631,6 +1631,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
> >>>>>>>> 	rhp = rcu_cblist_dequeue(&ready_cbs);
> >>>>>>>> 	for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
> >>>>>>>> 		debug_rcu_head_unqueue(rhp);
> >>>>>>>> +		debug_rcu_head_callback(rhp);
> >>>>>>>> 		local_bh_disable();
> >>>>>>>> 		rhp->func(rhp);
> >>>>>>>> 		local_bh_enable();
> >>>>>>>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 
> >>>>>>>> b0b885e071fa8dc..b7f8c67c586cdc4 100644
> >>>>>>>> --- a/kernel/rcu/tasks.h
> >>>>>>>> +++ b/kernel/rcu/tasks.h
> >>>>>>>> @@ -478,6 +478,7 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
> >>>>>>>> 	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
> >>>>>>>> 	len = rcl.len;
> >>>>>>>> 	for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = 
> >>>>>>>> rcu_cblist_dequeue(&rcl)) {
> >>>>>>>> +		debug_rcu_head_callback(rhp);
> >>>>>>>> 		local_bh_disable();
> >>>>>>>> 		rhp->func(rhp);
> >>>>>>>> 		local_bh_enable();
> >>>>>>>> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c index 
> >>>>>>>> bb8f7d270f01747..56e9a5d91d97ec5 100644
> >>>>>>>> --- a/kernel/rcu/tiny.c
> >>>>>>>> +++ b/kernel/rcu/tiny.c
> >>>>>>>> @@ -97,6 +97,7 @@ static inline bool rcu_reclaim_tiny(struct rcu_head 
> >>>>>>>> *head)
> >>>>>>>>
> >>>>>>>> 	trace_rcu_invoke_callback("", head);
> >>>>>>>> 	f = head->func;
> >>>>>>>> +	debug_rcu_head_callback(head);
> >>>>>>>> 	WRITE_ONCE(head->func, (rcu_callback_t)0L);
> >>>>>>>> 	f(head);
> >>>>>>>> 	rcu_lock_release(&rcu_callback_map);
> >>>>>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 
> >>>>>>>> 15aaff3203bf2d0..ed93ddb8203d42c 100644
> >>>>>>>> --- a/kernel/rcu/tree.c
> >>>>>>>> +++ b/kernel/rcu/tree.c
> >>>>>>>> @@ -2088,6 +2088,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
> >>>>>>>> 		trace_rcu_invoke_callback(rcu_state.name, rhp);
> >>>>>>>>
> >>>>>>>> 		f = rhp->func;
> >>>>>>>> +		debug_rcu_head_callback(rhp);
> >>>>>>>> 		WRITE_ONCE(rhp->func, (rcu_callback_t)0L);
> >>>>>>>> 		f(rhp);
> >>>>>>>>
> >>>>>>>> --
> >>>>>>>> 2.25.1
> >>>>>>>
> >>>>>>> .
> >>>>>>>
> >>>>>>
> >>>>>> --
> >>>>>> Regards,
> >>>>>>   Zhen Lei
> >>>>> .
> >>>>>
> >>>>
> >>>> -- 
> >>>> Regards,
> >>>>   Zhen Lei
> >>> .
> >>>
> >>
> >> -- 
> >> Regards,
> >>   Zhen Lei
> > .
> > 
> 
> -- 
> Regards,
>   Zhen Lei

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