Date:   Wed, 16 Nov 2022 22:43:57 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     <paulmck@...nel.org>
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 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.

> 
> 							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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