sched_ext: Replace tryget_task_struct() with get_task_struct()

The tryget_task_struct() calls in scx_sub_disable(), scx_root_enable_workfn() and scx_sub_enable_workfn() can never fail at the points they're invoked: - scx_root_enable_workfn() iterates over scx_tasks under scx_tasks_lock and rq lock. sched_ext_dead() removes tasks from scx_tasks under the same scx_tasks_lock before put_task_struct_rcu_user() runs in finish_task_switch(). So any task observed in scx_tasks must have usage > 0; put_task_struct_rcu_user() hasn't been called and the delayed_put_task_struct() callback that decrements usage cannot have been queued. - scx_sub_disable() and scx_sub_enable_workfn() iterate via css_task_iter, which takes a reference on each task in css_task_iter_next() and holds it until the next iter_next() call, so usage > 0 is guaranteed by the iter itself. The actual filter for dead tasks is the SCX_TASK_DEAD check inside scx_task_iter_next_locked(), not tryget; tryget only fails on zero usage, a state that can't be reached for tasks visible to these iters. Commit b7d4b28db7da ("sched_ext: Use SCX_TASK_READY test instead of tryget_task_struct() during class switch") removed an analogous tryget in the class-switch loop. Convert the remaining tryget calls to plain get_task_struct() and update the comment in scx_root_enable_workfn() that suggested tasks could be observed with zero @usage waiting for an RCU grace period.

ARAndrea Righi committed 6 months ago