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

[Paul E. McKenney]

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.

This would allow vmalloc_dump_obj() to be called unconditionally.

Other thoughts?

							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