Re: [PATCH v2] rcu: Dump memory object info if callback function is invalid

From: Leizhen (ThunderTown)
Date: Fri Nov 11 2022 - 21:32:54 EST



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@xxxxxxxxxx>
>>>> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>>>> ---
>>>> 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:
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