Message-ID: <20221112060815.GC725751@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 11 Nov 2022 22:08:15 -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 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.

> > 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.

							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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