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
>
next prev parent 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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