Re: [PATCH v2 08/26] selftests/resctrl: Split measure_cache_vals()

[Reinette Chatre]

Hi Ilpo,

On 12/7/2023 6:32 AM, Ilpo Järvinen wrote:
> On Tue, 28 Nov 2023, Reinette Chatre wrote:
>> On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:

...
>>> -	/*
>>> -	 * Measure llc occupancy from resctrl.
>>> -	 */
>>> -	if (!strncmp(param->resctrl_val, CMT_STR, sizeof(CMT_STR))) {
>>> -		ret = get_llc_occu_resctrl(&llc_occu_resc);
>>> -		if (ret < 0)
>>> -			return ret;
>>> -		llc_value = llc_occu_resc;
>>> -	}
>>> -	ret = print_results_cache(param->filename, bm_pid, llc_value);
>>> -	if (ret)
>>> +	ret = print_results_cache(filename, bm_pid, llc_perf_miss);
>>> +	return ret;
>>> +}
>>
>> Perhaps print_results_cache() can be made to return negative error
>> and this just be "return print_results_cache(...)" and the function
>> comment be accurate?
> 
> I think, I'll just change all "return errno;" to "return -1" before this,
> however, one open question which impacts whether this is actually Fixes 
> class issue:
> 
> It seems that perror()'s manpage doesn't answer one important question, 
> whether it ifself can alter errno or not. The resctrl selftest code 
> assumes it doesn't but some evidence I came across says otherwise so doing 
> return errno; after calling perror() might not even be valid at all.
> 
> So I'm tempted to create an additional Fixes patch about the return change 
> into the front of the series.
> 

I would not trust errno to contain code of earlier calls after a call to perror().
If errno is needed I think it should be saved before calling perror(). Looking
at perror() at [1] I do not see an effort to restore errno before it returns,
and looking at the implementation of perror() there appears to be many
opportunities for errno to change.

Reinette

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/perror.c;h=51e621e332a5e2aa76ecefb3bcf325efb43b345f;hb=HEAD#l47