Date:   Sat, 12 Nov 2022 10:39:55 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     <paulmck@...nel.org>, "Zhang, Qiang1" <qiang1.zhang@...el.com>
CC:     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 10:32, 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.
> 
>>
>> 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:

Oh, perhaps no new function is needed, mem_dump_obj() itself prints
debugging information. I will try a mm patch first.

> 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.
> 
>>
>> 							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

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