THREADING.README
1 Vendor HAL Threading Model 2 ========================== 3 The vendor HAL service has two threads: 4 1. HIDL thread: This is the main thread which processes all the incoming HIDL 5 RPC's. 6 2. Legacy HAL event loop thread: This is the thread forked off for processing 7 the legacy HAL event loop (wifi_event_loop()). This thread is used to process 8 any asynchronous netlink events posted by the driver. Any asynchronous 9 callbacks passed to the legacy HAL API's are invoked on this thread. 10 11 Synchronization Concerns 12 ======================== 13 wifi_legacy_hal.cpp has a bunch of global "C" style functions to handle the 14 legacy callbacks. Each of these "C" style function invokes a corresponding 15 "std::function" version of the callback which does the actual processing. 16 The variables holding these "std::function" callbacks are reset from the HIDL 17 thread when they are no longer used. For example: stopGscan() will reset the 18 corresponding "on_gscan_*" callback variables which were set when startGscan() 19 was invoked. This is not thread safe since these callback variables are 20 accesed from the legacy hal event loop thread as well. 21 22 Synchronization Solution 23 ======================== 24 Adding a global lock seems to be the most trivial solution to the problem. 25 a) All of the asynchronous "C" style callbacks will acquire the global lock 26 before invoking the corresponding "std::function" callback variables. 27 b) All of the HIDL methods will also acquire the global lock before processing 28 (in hidl_return_util::validateAndCall()). 29 30 Note: It's important that we only acquire the global lock for asynchronous 31 callbacks, because there is no guarantee (or documentation to clarify) that the 32 synchronous callbacks are invoked on the same invocation thread. If that is not 33 the case in some implementation, we will end up deadlocking the system since the 34 HIDL thread would have acquired the global lock which is needed by the 35 synchronous callback executed on the legacy hal event loop thread. 36
