Re: [PATCH] softirq: fix memory corruption when freeing tasklet_struct

[Allen]

> > There's a problem with the tasklet API - there is no reliable way how to
> > free a structure that contains tasklet_struct. The problem is that the
> > function tasklet_action_common calls task_unlock(t) after it called the
> > callback. If the callback does something that frees tasklet_struct,
> > task_unlock(t) would write into free memory.
>
> Ugh.
>
> I see what you're doing, but I have to say, I dislike this patch
> immensely. It feels like a serious misdesign that is then papered over
> with a hack.
>
> I'd much rather see us trying to move away from tasklets entirely in
> cases like this. Just say "you cannot do that".
>
 The idea of moving away from using tasklets has been discussed several times.
I am working on entirely moving away from using tasklets. Ofcourse, we have
some subsystems(like DMA), where we need to do a little more.

> In fact, of the two cases that want this new functionality, at least
> dm-verity already makes tasklets a conditional feature that isn't even
> enabled by default, and that was only introduced in the last couple of
> years.
>
> So I think dm-verity would be better off just removing tasklet use,
> and we should check whether there are better models for handling the
> latency issue.
>
> The dm-crypt.c case looks different, but similar. I'm not sure why it
> doesn't just use the workqueue for the "in interrupt" case. Like
> dm-verity, it already does have a workqueue option, and it's a
> setup-time option to say "don't use the workqueue for reads / writes".
> But it feels like the code should just say "tough luck, in interrupt
> context we *will* use workqueues".
>
> So honestly, both of the cases you bring up seem to be just BUGGY. The
> fix is not to extend tasklets to a new thing, the fix is to say "those
> two uses of tasklets were broken, and should go away".
>
> End result: I would suggest:
>
>  - just get rid of the actively buggy use of tasklets. It's not
> necessary in either case.
>
>  - look at introducing a "low-latency atomic workqueue" that looks
> *exactly* like a regular workqueue, but has the rule that it's per-cpu
> and functions on it cannot sleep
>
> because I think one common issue with workqueues - which are better
> designed than tasklets - is that scheduling latency.
>
> I think if we introduced a workqueue that worked more like a tasklet -
> in that it's run in softirq context - but doesn't have the interface
> mistakes of tasklets, a number of existing workqueue users might
> decide that that is exactly what they want.
>
> So we could have a per-cpu 'atomic_wq' that things can be scheduled
> on, and that runs from softirqs just like tasklets, and shares the
> workqueue queueing infrastructure but doesn't use the workqueue
> threads.
>
> Yes, the traditional use of workqueues is to be able to sleep and do
> things in process context, so that sounds a bit odd, but let's face
> it, we
>
>  (a) already have multiple classes of workqueues
>
>  (b) avoiding deep - and possibly recursive - stack depths is another
> reason people use workqueues
>
>  (c) avoiding interrupt context is a real concern, even if you don't
> want to sleep
>
> and I really *really* would like to get rid of tasklets entirely.
>
> They started as this very specific hardcoded softirq thing used by
> some drivers, and then the notion was generalized.
>
> And I think it was generalized badly, as shown by this example.
>
> I have added Tejun to the cc, so that he can throw his hands up in
> horror and say "Linus, you're crazy, your drug-fueled idea would be
> horrid because of Xyz".
>
> But *maybe* Tejun has been taking the same drugs I have, and goes
> "yeah, that would fit well".
>
> Tejun? Please tell me I'm not on some bad crack..
>
>                Linus
>
       - Allen