Re: WARN @lib/refcount.c:128 during hot unplug of I/O adapter.

[Tyrel Datwyler <turtle.in.the.kernel@gmail.com>]

On 04/11/2017 07:10 PM, Michael Ellerman wrote:
> Tyrel Datwyler <turtle.in.the.kernel@gmail.com> writes:
>> On 04/11/2017 02:00 AM, Michael Ellerman wrote:
>>> Tyrel Datwyler <tyreld@linux.vnet.ibm.com> writes:
>>>> I started looking at it when Bharata submitted a patch trying to fix the
>>>> issue for CPUs, but got side tracked by other things. I suspect that
>>>> this underflow has actually been an issue for quite some time, and we
>>>> are just now becoming aware of it thanks to the recount_t patchset being
>>>> merged.
>>>
>>> Yes I agree. Which means it might be broken in existing distros.
>>
>> Definitely. I did some profiling last night, and I understand the
>> hotplug case. It turns out to be as I suggested in the original thread
>> about CPUs. When the devicetree code was worked to move the tree out of
>> proc and into sysfs the sysfs detach code added a of_node_put to remove
>> the original of_init reference. pSeries Being the sole original
>> *dynamic* device tree user we had always issued a of_node_put in our
>> dlpar specific detach function to achieve that end. So, this should be a
>> pretty straight forward trivial fix.
> 
> Excellent, thanks.
> 
>> However, for the case where devices are present at boot it appears we a
>> leaking a lot of references resulting in the device nodes never actually
>> being released/freed after a dlpar remove. In the CPU case after boot I
>> count 8 more references taken than the hotplug case, and corresponding
>> of_node_put's are not called at dlpar remove time either. That will take
>> some time to track them down, review and clean up.

I found our reference leak. In topology_init() we call register_cpu()
for each possible logical cpu id. For any logical cpu present a
reference to the device node of the cpu core is grabbed and added to
cpu->dev.of_node. Which matches what I'm seeing on a Power8 lpar, 8
extraneous references which is equal to the 8 hardware threads of a core.

> 
> Yes that is a perennial problem unfortunately which we've never come up
> with a good solution for.
> 
> The (old) patch below might help track some of them down. I remember
> having a script to process the output of the trace and find mismatches,
> but I can't find it right now - but I'm sure you can hack up something
> :)

Haha, this patch is almost identical to what I hacked up Monday to get
an idea of where the refcounts were at. Probably wouldn't hurt to try
and upstream it into the driver/of tree.

-Tyrel

> 
> cheers
> 
> 
> diff --git a/arch/powerpc/include/asm/trace.h b/arch/powerpc/include/asm/trace.h
> index 32e36b16773f..ad32365082a0 100644
> --- a/arch/powerpc/include/asm/trace.h
> +++ b/arch/powerpc/include/asm/trace.h
> @@ -168,6 +168,44 @@ TRACE_EVENT(hash_fault,
>  		      __entry->addr, __entry->access, __entry->trap)
>  );
>  
> +TRACE_EVENT(of_node_get,
> +
> +	TP_PROTO(struct device_node *dn, int val),
> +
> +	TP_ARGS(dn, val),
> +
> +	TP_STRUCT__entry(
> +		__field(struct device_node *, dn)
> +		__field(int, val)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->dn = dn;
> +		__entry->val = val;
> +	),
> +
> +	TP_printk("get %d -> %d %s", __entry->val - 1, __entry->val, __entry->dn->full_name)
> +);
> +
> +TRACE_EVENT(of_node_put,
> +
> +	TP_PROTO(struct device_node *dn, int val),
> +
> +	TP_ARGS(dn, val),
> +
> +	TP_STRUCT__entry(
> +		__field(struct device_node *, dn)
> +		__field(int, val)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->dn = dn;
> +		__entry->val = val;
> +	),
> +
> +	TP_printk("put %d -> %d %s", __entry->val + 1, __entry->val, __entry->dn->full_name)
> +);
> +
>  #endif /* _TRACE_POWERPC_H */
>  
>  #undef TRACE_INCLUDE_PATH
> diff --git a/drivers/of/dynamic.c b/drivers/of/dynamic.c
> index c647bd1b6903..f5c3d761f3cd 100644
> --- a/drivers/of/dynamic.c
> +++ b/drivers/of/dynamic.c
> @@ -14,6 +14,8 @@
>  
>  #include "of_private.h"
>  
> +#include <asm/trace.h>
> +
>  /**
>   * of_node_get() - Increment refcount of a node
>   * @node:	Node to inc refcount, NULL is supported to simplify writing of
> @@ -23,8 +25,12 @@
>   */
>  struct device_node *of_node_get(struct device_node *node)
>  {
> -	if (node)
> +	if (node) {
>  		kobject_get(&node->kobj);
> +
> +		trace_of_node_get(node, atomic_read(&node->kobj.kref.refcount));
> +	}
> +
>  	return node;
>  }
>  EXPORT_SYMBOL(of_node_get);
> @@ -36,8 +42,10 @@ EXPORT_SYMBOL(of_node_get);
>   */
>  void of_node_put(struct device_node *node)
>  {
> -	if (node)
> +	if (node) {
>  		kobject_put(&node->kobj);
> +		trace_of_node_put(node, atomic_read(&node->kobj.kref.refcount));
> +	}
>  }
>  EXPORT_SYMBOL(of_node_put);
>  
>