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

[Steven Rostedt <rostedt@goodmis.org>]

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.

-- Steve