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    SubjectRe: [PATCH 1/2] tracing: Simplify & fix saved_tgids logic
    On Wed, Jun 30, 2021 at 8:31 AM Steven Rostedt <rostedt@goodmis.org> wrote:
    >
    > On Tue, 29 Jun 2021 17:34:05 -0700
    > Paul Burton <paulburton@google.com> wrote:
    >
    > > The tgid_map array records a mapping from pid to tgid, where the index
    > > of an entry within the array is the pid & the value stored at that index
    > > is the tgid.
    > >
    > > The saved_tgids_next() function iterates over pointers into the tgid_map
    > > array & dereferences the pointers which results in the tgid, but then it
    > > passes that dereferenced value to trace_find_tgid() which treats it as a
    > > pid & does a further lookup within the tgid_map array. It seems likely
    > > that the intent here was to skip over entries in tgid_map for which the
    > > recorded tgid is zero, but instead we end up skipping over entries for
    > > which the thread group leader hasn't yet had its own tgid recorded in
    > > tgid_map.
    > >
    > > A minimal fix would be to remove the call to trace_find_tgid, turning:
    > >
    > > if (trace_find_tgid(*ptr))
    > >
    > > into:
    > >
    > > if (*ptr)
    > >
    > > ..but it seems like this logic can be much simpler if we simply let
    > > seq_read() iterate over the whole tgid_map array & filter out empty
    > > entries by returning SEQ_SKIP from saved_tgids_show(). Here we take that
    > > approach, removing the incorrect logic here entirely.
    > >
    > > Signed-off-by: Paul Burton <paulburton@google.com>
    > > Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
    > > Cc: Steven Rostedt <rostedt@goodmis.org>
    > > Cc: Ingo Molnar <mingo@redhat.com>
    > > Cc: Joel Fernandes <joelaf@google.com>
    > > Cc: <stable@vger.kernel.org>
    > > ---
    >
    > Joel,
    >
    > Can you review this please.

    Sure thing Steve, will review it today.

    thanks,
    -Joel

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