Re: [PATCH] dma-debug: fix debug_dma_assert_idle(), use rcu_read_lock()

[Christoph Hellwig]

On Thu, Aug 13, 2020 at 12:02:41PM -0700, Linus Torvalds wrote:
> Yeah, that's ridiculously expensive, and serializes things for no good reason.
> 
> Your patch looks obviously correct to me (Christoph?),

It also looks correct to me.

> but it also
> makes me go "why are we doing this in the first place"?
> 
> Because it looks to me like
>  (a) the debug check is wrong
>  (b) this is left-over from early debugging
> 
> In particular, I don't see why we couldn't do a COW on a page that is
> under writeback at the same time. We're not changing the page that is
> doing DMA.

Yes.  We don't need to check for a DMA to the device, but a DMA from
the device while under DMA obviously is bogus.  But then again you'd
need to try really hard to do that.

> In fact, the whole "COW with DMA" makes me feel like the real bug may
> have been due that whole "ambiguous COW" thing, which was fixed in
> 17839856fd58 ("gup: document and work around "COW can break either
> way" issue")
> 
> That debug thing goes back almost 7 years, and I don't think it has
> caught anything in those seven years, but I could be wrong.
> 
> The commit that adds it does talk about a bug, but that code was
> removed entirely eventually. And google shows no hits for
> debug_dma_assert_idle() since - until your email.
> 
> So my gut feel is that we should remove the check entirely, although
> your patch does seem like a big improvement.
> 
> Christoph?

The whole thing predates my involvement with the code, but I defintively
think the patch from Hugh is a major improvement.  But I would also
have no problem with just removing it entirely.