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If a device uses blk_inc/dec_in_flight() in order to build macro operations that involve multiple requests for the block layer and that need to be completed as a unit before the BlockBackend can be considered drained, it sets the stage for a deadlock: When a drain is requested, the inner request at the BlockBackend level will be queued in blk_wait_while_drained() and wait until the drained section ends, but at the same time, drain_begin can only return if the whole macro operation at the device level has completed. Introduce a new interface to allow implementing the logic correctly: Instead of queueing individual requests, blk_co_start_request() calls blk_wait_while_drained() once at the beginning. The individual requests must then set BDRV_REQ_NO_QUEUE to avoid being queued and running into the deadlock; being wrapped in blk_co_start/end_request() makes sure that drain_begin waits for them and they don't sneak in when the BlockBackend is supposed to already be quiescent. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Message-ID: <20260421161132.99878-3-kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>