On Fri 2022-07-22 15:01:06, Rik van Riel wrote: > v2: a better approach, suggested by Petr (thank you) > ---8<--- > > When a KLP fails to apply, klp_reverse_transition will clear the > TIF_PATCH_PENDING flag on all tasks, except for newly created tasks > which are not on the task list yet. > > Meanwhile, fork will copy over the TIF_PATCH_PENDING flag from the > parent to the child early on, in dup_task_struct -> setup_thread_stack. > > Much later, klp_copy_process will set child->patch_state to match > that of the parent. > > However, the parent's patch_state may have been changed by KLP loading > or unloading since it was initially copied over into the child. > > This results in the KLP code occasionally hitting this warning in > klp_complete_transition: > > for_each_process_thread(g, task) { > WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > task->patch_state = KLP_UNDEFINED; > } > > This patch will set, or clear, the TIF_PATCH_PENDING flag in the child > process depending on whether or not it is needed at the time > klp_copy_process is called, at a point in copy_process where the > tasklist_lock is held exclusively, preventing races with the KLP > code. > > This should prevent this warning from triggering again in the > future. > > I have not yet figured out whether this would also help with races in > the other direction, where the child process fails to have TIF_PATCH_PENDING > set and somehow misses a transition, or whether the retries in > klp_try_complete_transition would catch that task and help it transition > later. It should fix these races as well. Both task->patch_state and TIF_PATCH_PENDING flag are almost always modified under tasklist_lock. One exception is klp_update_patch_state(current) but it could not race because klp_copy_process() is called under spinlock. So that "current" can't sleep and can't get migrated in the middle of klp_copy_process(). Another exception is klp_check_and_switch_task() that is called under p->pi_lock. It prevents rescheduling and the task will be migrated only when sleeping. As a result "current" again can't get migrated inside klp_copy_process(). Finally, the state of "idle" tasks (idle_task(cpu)) is updated without tasklist_lock. But they are not forked so that we are on safe side. > Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx> > Reported-by: Breno Leitao <leitao@xxxxxxxxxx> We should update the commit message and mention also the other two locations where the state is manipulated without tasklist_lock. I am sorry that I did not mention it on Friday. Also we should remove "I have not figured yet whether". The patch should fix these races as well. With the above changes: Reviewed-by: Petr Mladek <pmladek@xxxxxxxx> Best Regards, Petr