Re: [GIT pull] timers/urgent for v5.16-rc1

[Linus Torvalds]

On Sun, Nov 14, 2021 at 5:31 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> +       /*
> +        * A copied work entry from the old task is not meaningful, clear it.
> +        * N.B. init_task_work will not do this.
> +        */
> +       memset(&p->posix_cputimers_work.work, 0,
> +              sizeof(p->posix_cputimers_work.work));
> +       init_task_work(&p->posix_cputimers_work.work,
> +                      posix_cpu_timers_work);

Ugh.

Instead of the added four lines of comment, and two lines of
"memset()", maybe this should just have made init_task_work() DTRT?

Yes,. I see this:

        /* Protect against double add, see task_tick_numa and task_numa_work */
        p->numa_work.next               = &p->numa_work;
        ...
        init_task_work(&p->numa_work, task_numa_work);

but I think that one is so subtle and such a special case that it
should have been updated - just make that magic special flag happen
after the init_task_work.

A lot of the other cases seem to zero-initialize things elsewhere
(generally with kzalloc()), but I note that at least
io_ring_exit_work() seems to have this:

        struct io_tctx_exit exit;
        ...
        init_task_work(&exit.task_work, io_tctx_exit_cb);

and the ->next pointer is never set to NULL.

Now, in 99% of all cases the ->next pointer simply doesn't matter,
because task_work_add() will only set it, not caring about the old
value.

But apparently it matters for posix_cputimers_work and for numa_work,
and so I think it's very illogical that init_task_work() will not
actually initialize it properly.

Hmm?

I've pulled this, but it really looks like the wrong solution to the
whole "uninitialized data".

And that task_tick_numa() special case is truly horrendous, and really
should go after the init_task_work() regardless, exactly because you'd
expect that init_task_work() to initialize the work even if it doesn't
happen to right now.

Or is somebody doing init_task_work() to only change the work-function
on an already initialized work entry? Becuase that sounds both racy
and broken to me, and none of the things I looked at from a quick grep
looked like that at all.

              Linus