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References: <20211213232224.2023465-1-namhyung@kernel.org>
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From:   Marco Elver <elver@google.com>
Date:   Mon, 10 Jan 2022 09:58:23 +0100
Message-ID: <CANpmjNPuDUwrLH9GKEs6BgeRiL6uZ_tqa5NX_6J83E121+CUqg@mail.gmail.com>
Subject: Re: [PATCH v3] perf/core: Fix cgroup event list management
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Andi Kleen <ak@linux.intel.com>,
        Ian Rogers <irogers@google.com>
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On Tue, 14 Dec 2021 at 00:22, Namhyung Kim <namhyung@kernel.org> wrote:
>
> The active cgroup events are managed in the per-cpu cgrp_cpuctx_list.
> This list is only accessed from current cpu and not protected by any
> locks.  But from the commit ef54c1a476ae ("perf: Rework
> perf_event_exit_event()"), it's possible to access (actually modify)
> the list from another cpu.
>
> In the perf_remove_from_context(), it can remove an event from the
> context without an IPI when the context is not active.  This is not
> safe with cgroup events which can have some active events in the
> context even if ctx->is_active is 0 at the moment.  The target cpu
> might be in the middle of list iteration at the same time.
>
> If the event is enabled when it's about to be closed, it might call
> perf_cgroup_event_disable() and list_del() with the cgrp_cpuctx_list
> on a different cpu.
>
> This resulted in a crash due to an invalid list pointer access during
> the cgroup list traversal on the cpu which the event belongs to.
>
> Let's fallback to IPI to access the cgrp_cpuctx_list from that cpu.
> Similarly, perf_install_in_context() should use IPI for the cgroup
> events too.
>
> Cc: Marco Elver <elver@google.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

The final version needs:

Fixes: ef54c1a476ae ("perf: Rework perf_event_exit_event()")

so stable kernels will see it, unless this has already been picked up
in which case we need to email stable.

Thanks,
-- Marco