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 20:13:48 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     James Clark <james.clark@....com>
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 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.

Thanks,
Leo