Re: [PATCH v1 2/3] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}

James Clark
2021-08-23T16:00:24Z

On 23/08/2021 13:13, Leo Yan wrote:
> On Mon, Aug 23, 2021 at 11:57:52AM +0100, James Clark wrote:
>
> [...]
>
>> Ok thanks for the explanation, that makes sense now. I do have one other
>> point about the documentation for the function:
>
> Welcome!
>
>>> + * When update the AUX tail and detects any carrying in the high 32 bits, it
>>> + * means there have two store operations in user space and it cannot promise
>>> + * the atomicity for 64-bit write, so return '-1' in this case to tell the
>>> + * caller an overflow error has happened.
>>> + */
>>
>> I couldn't see how it can ever return -1, it seems like it would loop forever
>> until it reads the correct value.
>
> I use this chunk comment to address the function
> compat_auxtrace_mmap__write_tail():
>
> +int __weak compat_auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail)
> +{
> + struct perf_event_mmap_page *pc = mm->userpg;
> + u64 mask = (u64)(UINT32_MAX) << 32;
> +
> + if (tail & mask)
> + return -1;
> +
> + /* Ensure all reads are done before we write the tail out */
> + smp_mb();
> + WRITE_ONCE(pc->aux_tail, tail);
> + return 0;
> +}
>
> Please let me know if this is okay or not? Otherwise, if you think
> the format can cause confusion, I'd like to split the comments into
> two sections, one section for reading AUX head and another is for
> writing AUX tail.

I see what you mean now, I think keeping it in one section is fine, I would just
replace "When update the AUX tail and detects" with "When
compat_auxtrace_mmap__write_tail() detects". If the function name is there then
it's clear that it's not the return value of compat_auxtrace_mmap__read_head()

Thanks
James

>
> Thanks,
> Leo
>