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

[Ravi Bangoria]

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.

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).

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

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.

Thanks,
Ravi