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From: James Clark <james.clark@arm.com>
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
Subject: Re: [PATCH v1 2/3] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}
Date: Mon, 23 Aug 2021 17:00:14 +0100	[thread overview]
Message-ID: <1c0101fd-2ca6-4741-5d72-82d3e00bfeb6@arm.com> (raw)
In-Reply-To: <20210823121348.GE100516@leoy-ThinkPad-X240s>



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
> 

  reply	other threads:[~2021-08-23 16:00 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-09 11:27 [PATCH v1 0/3] perf: Support compat mode for AUX ring buffer Leo Yan
2021-08-09 11:27 ` [PATCH v1 1/3] perf env: Track kernel 64-bit mode in environment Leo Yan
2021-08-09 20:11   ` Arnaldo Carvalho de Melo
2021-08-09 11:27 ` [PATCH v1 2/3] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail} Leo Yan
2021-08-09 20:12   ` Arnaldo Carvalho de Melo
2021-08-12  9:23   ` Adrian Hunter
2021-08-13 16:22   ` James Clark
2021-08-23  9:51     ` Leo Yan
2021-08-23 10:57       ` James Clark
2021-08-23 12:13         ` Leo Yan
2021-08-23 16:00           ` James Clark [this message]
2021-08-24  2:13             ` Leo Yan
2021-08-09 11:27 ` [PATCH v1 3/3] perf auxtrace arm: Support compat_auxtrace_mmap__{read_head|write_tail} Leo Yan
2021-08-23 12:23   ` James Clark
2021-08-23 13:30     ` Leo Yan
2021-08-23 13:39       ` Russell King (Oracle)
2021-08-23 14:54         ` Leo Yan

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