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Subject: Re: [PATCH v1 2/3] perf auxtrace: Add
 compat_auxtrace_mmap__{read_head|write_tail}
To:     Leo Yan <leo.yan@linaro.org>
Cc:     linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Will Deacon <will@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Mike Leach <mike.leach@linaro.org>,
        John Garry <john.garry@huawei.com>,
        Andi Kleen <ak@linux.intel.com>,
        Riccardo Mancini <rickyman7@gmail.com>,
        Jin Yao <yao.jin@linux.intel.com>,
        Li Huafei <lihuafei1@huawei.com>, coresight@lists.linaro.org
References: <20210809112727.596876-1-leo.yan@linaro.org>
 <20210809112727.596876-3-leo.yan@linaro.org>
 <2b4e0c07-a8df-cca6-6a94-328560f4b0c6@arm.com>
 <20210823095155.GC100516@leoy-ThinkPad-X240s>
 <319ee11a-06f7-abde-6495-d2175928b9fe@arm.com>
 <20210823121348.GE100516@leoy-ThinkPad-X240s>
From:   James Clark <james.clark@arm.com>
Message-ID: <1c0101fd-2ca6-4741-5d72-82d3e00bfeb6@arm.com>
Date:   Mon, 23 Aug 2021 17:00:14 +0100
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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
>