linux-kernel - Re: [PATCH v3 1/3] perf pmu: Move pmu__find_core

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Date:   Thu, 14 Sep 2023 07:38:24 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     James Clark <james.clark@....com>,
        linux-perf-users@...r.kernel.org, irogers@...gle.com,
        acme@...nel.org
Cc:     Will Deacon <will@...nel.org>, Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Haixin Yu <yuhaixin.yhx@...ux.alibaba.com>,
        Jing Zhang <renyu.zj@...ux.alibaba.com>,
        Eduard Zingerman <eddyz87@...il.com>,
        Ravi Bangoria <ravi.bangoria@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] perf pmu: Move pmu__find_core_pmu() to pmus.c

On 13/09/2023 16:33, James Clark wrote:
> pmu__find_core_pmu() more logically belongs in pmus.c because it
> iterates over all PMUs, so move it to pmus.c
> 
> At the same time rename it to perf_pmus__find_core_pmu() to match the
> naming convention in this file.
> 
> list_prepare_entry() can't be used in perf_pmus__scan_core() anymore now
> that it's called from the same compilation unit. This is with -O2
> (specifically -O1 -ftree-vrp -finline-functions
> -finline-small-functions) which allow the bounds of the array
> access to be determined at compile time. list_prepare_entry() subtracts
> the offset of the 'list' member in struct perf_pmu from &core_pmus,
> which isn't a struct perf_pmu. The compiler sees that pmu results in
> &core_pmus - 8 and refuses to compile. At runtime this works because
> list_for_each_entry_continue() always adds the offset back again before
> dereferencing ->next, but it's technically undefined behavior. With
> -fsanitize=undefined an additional warning is generated.
> 
> Using list_first_entry_or_null() to get the first entry here avoids
> doing &core_pmus - 8 but has the same result and fixes both the compile
> warning and the undefined behavior warning. There are other uses of
> list_prepare_entry() in pmus.c, but the compiler doesn't seem to be
> able to see that they can also be called with &core_pmus, so I won't
> change any at this time.
> 
> Signed-off-by: James Clark<james.clark@....com>
> ---


Reviewed-by: John Garry <john.g.garry@...cle.com>