RE: [PATCH v1 4/6] perf threads: Move threads to its own files

[David Laight <David.Laight@ACULAB.COM>]

From: Ian Rogers
> Sent: 27 February 2024 07:24
> 
> On Mon, Feb 26, 2024 at 11:07 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Tue, Feb 13, 2024 at 10:37 PM Ian Rogers <irogers@google.com> wrote:
> > >
> > > Move threads out of machine and move thread_rb_node into the C
> > > file. This hides the implementation of threads from the rest of the
> > > code allowing for it to be refactored.
> > >
> > > Locking discipline is tightened up in this change.
> >
> > Doesn't look like a simple code move.  Can we split the locking
> > change from the move to make the reviewer's life a bit easier? :)
> 
> Not sure I follow. Take threads_nr as an example.
> 
> The old code is in machine.c, so:
> -static size_t machine__threads_nr(const struct machine *machine)
> -{
> -       size_t nr = 0;
> -
> -       for (int i = 0; i < THREADS__TABLE_SIZE; i++)
> -               nr += machine->threads[i].nr;
> -
> -       return nr;
> -}
> 
> The new code is in threads.c:
> +size_t threads__nr(struct threads *threads)
> +{
> +       size_t nr = 0;
> +
> +       for (int i = 0; i < THREADS__TABLE_SIZE; i++) {
> +               struct threads_table_entry *table = &threads->table[i];
> +
> +               down_read(&table->lock);
> +               nr += table->nr;
> +               up_read(&table->lock);
> +       }
> +       return nr;
> +}
> 
> So it is a copy paste from one file to the other. The only difference
> is that the old code failed to take a lock when reading "nr" so the
> locking is added. I wanted to make sure all the functions in threads.c
> were properly correct wrt locking, semaphore creation and destruction,
> etc.  We could have a broken threads.c and fix it in the next change,
> but given that's a bug it could make bisection more difficult.
> Ultimately I thought the locking changes were small enough to not
> warrant being on their own compared to the advantages of having a sane
> threads abstraction.

The lock is pretty much entirely pointless.
All it really does is slow the code down.
The most you could want is:
	nr += READ_ONCE(table->nr);
to avoid any hypothetical data tearing.

	David

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