On Fri, Mar 15, 2024 at 01:46:30PM +0100, Andreas Hindborg wrote: > Ming Lei <ming.lei@xxxxxxxxxx> writes: > > On Fri, Mar 15, 2024 at 08:52:46AM +0100, Andreas Hindborg wrote: > >> Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx> writes: > >> > >> > On Thu, Mar 14, 2024 at 8:23 PM Andreas Hindborg <nmi@xxxxxxxxxxxx> 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
