BACKGROUND
The exit_group syscall, which is implicitly called by libc after the main function
returns, kills all threads in a thread group, even if these threads are
running, sleeping, or waiting on a futex.
PROBLEM
In normal use cases, exit_group does nothing since a well-written program
should terminate all threads before the main function returns. But when this is
not the case, exit_group can clean up the mess.
Currently, Occlum does not implement exit_group. And the Occlum PAL process
waits for all tasks (i.e., SGX threads) to finish before exiting. So without
exit_group implemented, some tasks may be still running if after the main task
exits. And this causes the Occlum PAL process to wait---forever.
WORKAROUND
To implement a real exit_group, we need signals to kill threads. But we do not
have signals, yet. So we come up with a workaround: instead of waiting all
tasks to finish in PAL, we just wait for the main task. As soon as the main
task exits, the PAL process terminates, killing the remaining tasks.
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