Re: [PATCH v3 6/9] trace_uprobe: Support SDT markers having reference count (semaphore)

[Ravi Bangoria]

Hi Masami,

On 05/07/2018 09:26 PM, Masami Hiramatsu wrote:
> On Mon, 7 May 2018 13:51:21 +0530
> Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx> wrote:
>
>> Hi Masami,
>>
>> On 05/04/2018 07:51 PM, Ravi Bangoria wrote:
>>>>> +}
>>>>> +
>>>>> +static void sdt_increment_ref_ctr(struct trace_uprobe *tu)
>>>>> +{
>>>>> +	struct uprobe_map_info *info;
>>>>> +
>>>>> +	uprobe_down_write_dup_mmap();
>>>>> +	info = uprobe_build_map_info(tu->inode->i_mapping,
>>>>> +				tu->ref_ctr_offset, false);
>>>>> +	if (IS_ERR(info))
>>>>> +		goto out;
>>>>> +
>>>>> +	while (info) {
>>>>> +		down_write(&info->mm->mmap_sem);
>>>>> +
>>>>> +		if (sdt_find_vma(tu, info->mm, info->vaddr))
>>>>> +			sdt_update_ref_ctr(info->mm, info->vaddr, 1);
>>>> Don't you have to handle the error to map pages here?
>>> Correct.. I think, I've to feedback error code to probe_event_{enable|disable}
>>> and handler failure there.
>> I looked at this. Actually, It looks difficult to feedback errors to
>> probe_event_{enable|disable}, esp. in the mmap() case.
> Hmm, can't you roll that back if sdt_increment_ref_ctr() fails?
> If so, how does sdt_decrement_ref_ctr() work in that case?

Yes, it's easy to rollback in sdt_increment_ref_ctr(). But not much can
be done if trace_uprobe_mmap() fails.

What would be good is, if we can feedback uprobe_mmap() failures
to the perf infrastructure, which can finally be parsed by perf record.
But that should be done as a separate work.

>> Is it fine if we just warn sdt_update_ref_ctr() failures in dmesg? I'm
>> doing this in [PATCH 7]. (Though, it makes more sense to do that in
>> [PATCH 6], will change it in next version).
> Of course we need to warn it at least, but the best is rejecting to
> enable it.

Yes, we can reject it for sdt_increment_ref_ctr() failures.

>> Any better ideas?
>>
>> BTW, same issue exists for normal uprobe. If uprobe_mmap() fails,
>> there is no feedback to trace_uprobe and no warnigns in dmesg as
>> well !! There was a patch by Naveen to warn such failures in dmesg
>> but that didn't go in: https://lkml.org/lkml/2017/9/22/155
> Oops, that's a real bug. It seems the ball is in Naveen's hand.
> Naveen, could you update it according to Oleg's comment, and resend it?
>
>> Also, I'll add a check in sdt_update_ref_ctr() to make sure reference
>> counter never goes to negative incase increment fails but decrement
>> succeeds. OTOH, if increment succeeds but decrement fails, the
>> counter remains >0 but there is no harm as such, except we will
>> execute some unnecessary code.
> I see. Please carefully clarify whether such case is kernel's bug or not.
> I would like to know what the condition causes that uneven behavior.

Sure, will do that.

Thanks,
Ravi