linux-kernel - Re: [PATCH v1 2/3] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write

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Date:   Mon, 23 Aug 2021 17:00:14 +0100
From:   James Clark <james.clark@....com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        John Garry <john.garry@...wei.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Riccardo Mancini <rickyman7@...il.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Li Huafei <lihuafei1@...wei.com>, coresight@...ts.linaro.org
Subject: Re: [PATCH v1 2/3] perf auxtrace: Add
 compat_auxtrace_mmap__{read_head|write_tail}



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
>