Date:   Mon, 14 Nov 2022 15:18:10 +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 2022/11/12 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)

> 
>>> 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.

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

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