Re: [PATCH v2 1/2] tracing: Simplify & fix saved_tgids logic

[Joel Fernandes]

On Thu, Jul 1, 2021 at 1:24 PM Paul Burton <paulburton@xxxxxxxxxx> 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@xxxxxxxxxx>
> Fixes: d914ba37d714 ("tracing: Add support for recording tgid of tasks")
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Joel Fernandes <joelaf@xxxxxxxxxx>

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>