Re: [PATCH v9 0/5] rust: adds Bitmap API, ID pool and bindings

[Burak Emir]

+Carlos

On Tue, May 27, 2025 at 4:43 PM Yury Norov <yury.norov@xxxxxxxxx> wrote:
>
> On Mon, May 26, 2025 at 03:01:29PM +0000, Burak Emir wrote:
> > We include set_bit_atomic and clear_bit_atomic operations. One has
> > to avoid races with non-atomic operations, which is ensure by the
> > Rust type system: either callers have shared references &bitmap in
> > which case the mutations are atomic operations. Or there is a
> > exclusive reference &mut bitmap, in which case there is no concurrent
> > access.
>
> Here I'm lost. In the other email you say:
>
> > You also commented on the API. The weirdness of the API is all due to
> > the separating "request to shrink/grow" from allocation.
> > Since allocation can happen while other threads may mess with the id
> > pool, one has to double check that the request to shrink/grow still
> > makes sense.
>
> And here you say:
>
> > there is a
> > exclusive reference &mut bitmap, in which case there is no concurrent
> > access
>
> So to me it sounds like if I want to resize, I just allocate a new bitmap,
> take this exclusive reference, copy IDs, swap the pointers in
> corresponding class, and that's it. What did I miss?

The two emails are about two related but different things:
1. the bitmap API which uses kmalloc with GFP_KERNEL, and what you
suggest could be done with &mut references
2. the id_pool and its fine-grained API that permits controlling the
time of allocation, which is used in binder which relies on a
spinlock.

My limited understanding is that calling kmalloc w/ GFP_KERNEL while
holding a spinlock is not good, since the kmalloc may sleep.
You can see this "first unlock, then alloc, then lock again" pattern
in binder code here:
https://github.com/torvalds/linux/blob/15d9da3f818cae676f822a04407d3c17b53357d2/drivers/android/binder.c#L1099

cheers,
- Burak