Re: [PATCHv7 2/2] watchdog/softlockup: report the most frequent interrupts

[Bitao Hu <yaoma@linux.alibaba.com>]

Hi,

On 2024/2/15 19:30, Thomas Gleixner wrote:
> On Wed, Feb 14 2024 at 10:14, Bitao Hu wrote:
>> +static void start_counting_irqs(void)
>> +{
>> +	int i;
>> +	int local_nr_irqs;
>> +	struct irq_desc *desc;
>> +	u32 *counts = __this_cpu_read(hardirq_counts);
>> +
>> +	if (!counts) {
>> +		/*
>> +		 * nr_irqs has the potential to grow at runtime. We should read
>> +		 * it and store locally to avoid array out-of-bounds access.
>> +		 */
>> +		local_nr_irqs = nr_irqs;
>> +		counts = kcalloc(local_nr_irqs, sizeof(u32), GFP_ATOMIC);
> 
> Seriously? The system has a problem and you allocate memory from the
> detection code in hard interrupt context?
I want all the changes for this feature to be concentrated within the
watchdog module, and I am also unsure whether modifying the irq code
for this feature would be justified. Hence, I opted for this approach.
However, your reply on V1 demonstrated the proper way to do it, so I
will refactor accordingly.

>> +		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
>> +			if (irq_counts_sorted[i].irq == -1)
>> +				break;
>> +
>> +			desc = irq_to_desc(irq_counts_sorted[i].irq);
>> +			if (desc && desc->action)
>> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
>> +				       i + 1, irq_counts_sorted[i].counts,
>> +				       irq_counts_sorted[i].irq, desc->action->name);
> 
> You cannot dereference desc->action here:
> 
>      1) It can be NULL'ed between check and dereference.
> 
>      2) Both 'action' and 'action->name' can be freed in parallel
> 
> And no, you cannot take desc->lock here to prevent this. Stop fiddling
> in the internals of interrupt descriptors.
Thanks for your analysis. However, I have a question. 'action->name'
cannot be accessed here, and it seems that merely outputting the
irq number provides insufficient information?

> 
> 
> and the analysis boils down to:
> 
>          u64 cnt, sorted[3] = {};
>          unsigned int irq, i;
> 
>      	for_each_active_irq(irq) {
>          	cnt = kstat_get_irq_since_snapshot(irq);
> 
> 		if (cnt) {
>                       	for (cnt = (cnt << 32) + irq, i = 0; i < 3; i++) {
>                  		if (cnt > sorted[i])
>                          		swap(cnt, sorted[i]);
Hmm, I think the approach here isn't optimal. If some interrupts
have the same count, then it effectively results in sorting by the
irq number. Is my understanding correct?

Best Regards,
	Bitao