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Date: Fri, 15 Mar 2024 23:24:23 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Andreas Hindborg <nmi@metaspace.dk>
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Subject: Re: [RFC PATCH 1/5] rust: block: introduce `kernel::block::mq` module
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On Fri, Mar 15, 2024 at 01:46:30PM +0100, Andreas Hindborg wrote:
> Ming Lei <ming.lei@redhat.com> writes:
> > On Fri, Mar 15, 2024 at 08:52:46AM +0100, Andreas Hindborg wrote:
> >> Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> writes:
> >> 
> >> > On Thu, Mar 14, 2024 at 8:23 PM Andreas Hindborg <nmi@metaspace.dk> wrote:
> >> >>
> >> >> The way the current code compiles, <kernel::block::mq::Request as
> >> >> kernel::types::AlwaysRefCounted>::dec_ref` is inlined into the `rnull`
> >> >> module. A relocation for `rust_helper_blk_mq_free_request_internal`
> >> >> appears in `rnull_mod.ko`. I didn't test it yet, but if
> >> >> `__blk_mq_free_request` (or the helper) is not exported, I don't think
> >> >> this would be possible?
> >> >
> >> > Yeah, something needs to be exported since there is a generic
> >> > involved, but even if you want to go the route of exporting only a
> >> > different symbol, you would still want to put it in the C header so
> >> > that you don't get the C missing declaration warning and so that we
> >> > don't have to write the declaration manually in the helper.
> >> 
> >> That is what I did:
> >> 
> >> @@ -703,6 +703,7 @@ int blk_mq_alloc_sq_tag_set(struct blk_mq_tag_set *set,
> >>  		unsigned int set_flags);
> >>  void blk_mq_free_tag_set(struct blk_mq_tag_set *set);
> >>  
> >> +void __blk_mq_free_request(struct request *rq);
> >>  void blk_mq_free_request(struct request *rq);
> >>  int blk_rq_poll(struct request *rq, struct io_comp_batch *iob,
> >>  		unsigned int poll_flags);
> >
> > Can you explain in detail why one block layer internal helper is
> > called into rnull driver directly? It never happens in C driver code.
> 
> It is not the rust null block driver that calls this symbol directly. It
> is called by the Rust block device driver API. But because of inlining,
> the symbol is referenced from the loadable object.

What is the exact Rust block device driver API? The key point is that how
the body of one exported kernel C API(EXPORT_SYMBOL) becomes inlined
with Rust driver.

> 
> The reason we have to call this symbol directly is to ensure proper
> lifetime of the `struct request`. For example in C, when a driver

Sounds Rust API still calls into __blk_mq_free_request() directly, right?

If that is the case, the usecase need to be justified, and you need
to write one standalone patch with the exact story for exporting
__blk_mq_free_request().

> converts a tag to a request, the developer makes sure to only ask for
> requests which are outstanding in the driver. In Rust, for the API to be
> sound, we must ensure that the developer cannot write safe code that
> obtains a reference to a request that is not owned by the driver.
> 
> A similar issue exists in the null block driver when timer completions
> are enabled. If the request is cancelled and the timer fires after the
> request has been recycled, there is a problem because the timer holds a
> reference to the request private data area.
> 
> To that end, I use the `atomic_t ref` field of the C `struct request`
> and implement the `AlwaysRefCounted` Rust trait for the request type.
> This is a smart pointer that owns a reference to the pointee. In this
> way, the request is not freed and recycled until the smart pointer is
> dropped. But if the smart pointer holds the last reference when it is
> dropped, it must be able to free the request, and hence it has to call
> `__blk_mq_free_request`.

For callbacks(queue_rq, timeout, complete) implemented by driver, block
layer core guaranteed that the passed request reference is live.

So driver needn't to worry about request lifetime, same with Rust
driver, I think smart pointer isn't necessary for using request in
Rust driver.


Thanks, 
Ming