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

[Joel Fernandes]

On Wed, Jun 30, 2021 at 8:31 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Tue, 29 Jun 2021 17:34:05 -0700
> 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>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > ---
>
> Joel,
>
> Can you review this please.

Sure thing Steve, will review it today.

thanks,
-Joel