841a1fd579
The kernel provides a "recall" feature issued on threads to force a thread into an exception. In the exception the current state of the thread can be obtained and its execution can be halted/paused. However, the recall exception is only delivered when the next time the thread would leave the kernel. That means the delivery is asynchronous and Genode has to wait until the exception triggered. Waiting for the exception can either be done in the cpu_session service or outside the service in the protection domain of the caller. It turned out that waiting inside the cpu_service is prone to deadlock the system. The cpu_session interface is one of many session interfaces handled by the same thread inside Core. Deadlock situation: * The caller (thread_c) to pause some thread_p manages to establish the call to the cpu_session thread_s of Core but get be interrupted before issuing the actual pause (recall) command. * Now the - to be recalled thread_p - is scheduled and tries to invoke another service of Core, like making log output. * Since the Core thread_s is handling the session request of thread_c, the kernel uses the timeslice of thread_p to help to finish the request handled by thread_s. * Thread_s issues the actual pause/recall on thread_p and blocks inside Core to wait for the recall exception to be issued. * thread_p will leave not the kernel before finishing it actual IPC with thread_s which is blocked waiting for thread_p. That is the reason why the waiting/blocking for the recall exception taking place must be done on NOVA in the context of the caller (thread_1). Introduce a pause_sync call to the cpu_session which returns a semaphore capability to the caller. The caller blocks on the semaphore and is woken up when the pager of thread_p receives the recall exception with the state of thread_p. |
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| .. | ||
| 32bit/nova | ||
| 64bit/nova | ||
| base | ||
| cap_session | ||
| cpu_session | ||
| nova | ||
| nova_cpu_session | ||
| pd_session | ||
| rm_session | ||
| signal_session | ||