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/** @file
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/** @file
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 */
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 */
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/* IPC resources management
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/* IPC resources management
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 *
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 *
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 * The goal of this source code is to properly manage IPC resources
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 * The goal of this source code is to properly manage IPC resources and allow
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 * and allow straight and clean clean-up procedure upon task termination.
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 * straight and clean clean-up procedure upon task termination.
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 *
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 *
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 * The pattern of usage of the resources is:
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 * The pattern of usage of the resources is:
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 * - allocate empty phone slot, connect | deallocate slot
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 * - allocate empty phone slot, connect | deallocate slot
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 * - disconnect connected phone (some messages might be on the fly)
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 * - disconnect connected phone (some messages might be on the fly)
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 * - find phone in slot and send a message using phone
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 * - find phone in slot and send a message using phone
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 * - hangup phone (the caller has hung up)
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 * - hangup phone (the caller has hung up)
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 * - hangup phone (the answerbox is exiting)
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 * - hangup phone (the answerbox is exiting)
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 *
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 *
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 * Locking strategy
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 * Locking strategy
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 *
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 *
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 * - To use a phone, disconnect a phone etc., the phone must be
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 * - To use a phone, disconnect a phone etc., the phone must be first locked and
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 *   first locked and then checked that it is connected
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 *   then checked that it is connected
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 * - To connect an allocated phone it need not be locked (assigning
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 * - To connect an allocated phone it need not be locked (assigning pointer is
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 *   pointer is atomic on all platforms)
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 *   atomic on all platforms)
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 *
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 *
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 * - To find an empty phone slot, the TASK must be locked
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 * - To find an empty phone slot, the TASK must be locked
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 * - To answer a message, the answerbox must be locked
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 * - To answer a message, the answerbox must be locked
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 * - The locking of phone and answerbox is done at the ipc_ level.
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 * - The locking of phone and answerbox is done at the ipc_ level.
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 *   It is perfectly correct to pass unconnected phone to these functions
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 *   It is perfectly correct to pass unconnected phone to these functions and
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 *   and proper reply will be generated.
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 *   proper reply will be generated.
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 *
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 *
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 * Locking order
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 * Locking order
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 *
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 *
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 * - first phone, then answerbox
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 * - first phone, then answerbox
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 *   + Easy locking on calls
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 *   + Easy locking on calls
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 *   - Very hard traversing list of phones when disconnecting because
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 *   - Very hard traversing list of phones when disconnecting because the phones
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 *     the phones may disconnect during traversal of list of connected phones.
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 *     may disconnect during traversal of list of connected phones. The only
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 *     The only possibility is try_lock with restart of list traversal.
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 *     possibility is try_lock with restart of list traversal.
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 *
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 *
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 * Destroying is less frequent, this approach is taken.
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 * Destroying is less frequent, this approach is taken.
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 *
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 *
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 * Phone call
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 * Phone call
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 *
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 *
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 * *** Connect_me_to ***
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 * *** Connect_me_to ***
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 * The caller sends IPC_M_CONNECT_ME_TO to an answerbox. The server
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 * The caller sends IPC_M_CONNECT_ME_TO to an answerbox. The server receives
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 * receives 'phoneid' of the connecting phone as an ARG3. If it answers
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 * 'phoneid' of the connecting phone as an ARG3. If it answers with RETVAL=0,
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 * with RETVAL=0, the phonecall is accepted, otherwise it is refused.
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 * the phonecall is accepted, otherwise it is refused.
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 *
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 *
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 * *** Connect_to_me ***
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 * *** Connect_to_me ***
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 * The caller sends IPC_M_CONNECT_TO_ME, with special
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 * The caller sends IPC_M_CONNECT_TO_ME.
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 * The server receives an automatically
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 * The server receives an automatically opened phoneid. If it accepts
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 * opened phoneid. If it accepts (RETVAL=0), it can use the phoneid
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 * (RETVAL=0), it can use the phoneid immediately.
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 * immediately.
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 * Possible race condition can arise, when the client receives messages
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 * Possible race condition can arise, when the client receives messages from new
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 * from new connection before getting response for connect_to_me message.
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 * connection before getting response for connect_to_me message. Userspace
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 * Userspace should implement handshake protocol that would control it.
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 * should implement handshake protocol that would control it.
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 *
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 *
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 * Phone hangup
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 * Phone hangup
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 *
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 *
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 * *** The caller hangs up (sys_ipc_hangup) ***
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 * *** The caller hangs up (sys_ipc_hangup) ***
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 * - The phone is disconnected (no more messages can be sent over this phone),
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 * - The phone is disconnected (no more messages can be sent over this phone),
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 *   all in-progress messages are correctly handled. The anwerbox receives
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 *   all in-progress messages are correctly handled. The answerbox receives
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 *   IPC_M_PHONE_HUNGUP call from the phone that hung up. When all async
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 *   IPC_M_PHONE_HUNGUP call from the phone that hung up. When all async
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 *   calls are answered, the phone is deallocated.
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 *   calls are answered, the phone is deallocated.
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 *
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 *
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 * *** The answerbox hangs up (ipc_answer(EHANGUP))
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 * *** The answerbox hangs up (ipc_answer(EHANGUP))
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 * - The phone is disconnected. EHANGUP response code is sent
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 * - The phone is disconnected. EHANGUP response code is sent