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a4d8837553
Add UBLK_U_IO_FETCH_IO_CMDS command to enable efficient batch processing of I/O requests. This multishot uring_cmd allows the ublk server to fetch multiple I/O commands in a single operation, significantly reducing submission overhead compared to individual FETCH_REQ* commands. Key Design Features: 1. Multishot Operation: One UBLK_U_IO_FETCH_IO_CMDS can fetch many I/O commands, with the batch size limited by the provided buffer length. 2. Dynamic Load Balancing: Multiple fetch commands can be submitted simultaneously, but only one is active at any time. This enables efficient load distribution across multiple server task contexts. 3. Implicit State Management: The implementation uses three key variables to track state: - evts_fifo: Queue of request tags awaiting processing - fcmd_head: List of available fetch commands - active_fcmd: Currently active fetch command (NULL = none active) States are derived implicitly: - IDLE: No fetch commands available - READY: Fetch commands available, none active - ACTIVE: One fetch command processing events 4. Lockless Reader Optimization: The active fetch command can read from evts_fifo without locking (single reader guarantee), while writers (ublk_queue_rq/ublk_queue_rqs) use evts_lock protection. The memory barrier pairing plays key role for the single lockless reader optimization. Implementation Details: - ublk_queue_rq() and ublk_queue_rqs() save request tags to evts_fifo - __ublk_acquire_fcmd() selects an available fetch command when events arrive and no command is currently active - ublk_batch_dispatch() moves tags from evts_fifo to the fetch command's buffer and posts completion via io_uring_mshot_cmd_post_cqe() - State transitions are coordinated via evts_lock to maintain consistency Reviewed-by: Caleb Sander Mateos <csander@purestorage.com> Signed-off-by: Ming Lei <ming.lei@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>