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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Mon, 10 Jan 2022 11:49:22 -0800
Message-ID: <CAM9d7ci-xAWYJEspm8VXRy257vG9r7HDVuQ0Uoej49OG4f5qKQ@mail.gmail.com>
Subject: Re: [PATCH v3] perf/core: Fix cgroup event list management
To:     Marco Elver <elver@google.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     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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Hi Marco,

On Mon, Jan 10, 2022 at 12:58 AM Marco Elver <elver@google.com> wrote:
>
> 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.

Right, it should go to the stable tree.

Peter, do you want me to resend a new version?

Thanks,
Namhyung