The Infineon PSOC 6 BLESS HCI driver in drivers/bluetooth/hci/hci_infineon_psoc6_bless.c stored the outgoing HCI packet descriptor in a file-scope variable, static cy_stc_ble_hci_tx_packet_info_t hci_tx_pkt. psoc6_bless_send() cleared and populated that descriptor's packetType, dataLength and data fields before acquiring psoc6_bless_operation_sem, so the driver's serialising semaphore protected only the call into Cy_BLE_SoftHciSendAppPkt() and not the shared state that call reads.

The Bluetooth host does not guarantee a single HCI sender. Most transmissions are funnelled through the cooperative TX-processor work queue, but bt_hci_cmd_send() deliberately bypasses bt_dev.cmd_tx_queue for BT_HCI_OP_HOST_NUM_COMPLETED_PACKETS and calls bt_send() directly from the caller's thread; that caller is bt_hci_host_num_completed_packets(), the acl_in_pool destroy callback, which runs on whichever thread drops the last reference to a received ACL buffer. CONFIG_BT_HCI_ACL_FLOW_CONTROL defaults to y on this driver's boards, and an application that returns -EINPROGRESS from its L2CAP recv callback and later calls bt_l2cap_chan_recv_complete() performs that unref from its own preemptible thread. A remote peer that generates Bluetooth traffic concurrently with the device's own transmissions raises the collision rate.

When two senders interleave, the semaphore holder can hand the controller a descriptor whose dataLength comes from one net_buf and whose data pointer comes from another — or on…