On 25.06.19 09:42:14, Peter Zijlstra wrote: > On Mon, Jun 24, 2019 at 04:58:10PM -0400, Frank Ch. Eigler wrote: > > From peterz's comments, the patches, it's not obvious to me how one is > > to choose between 0 (NOTIFY_DONE) and 1 (NOTIFY_OK) in the case of a > > routine success. > > I'm not sure either; what I think I choice was: > > - if I want to completely ignore the callback, use DONE (per the > "Don't care" comment). > > - if we finished the notifier without error, use OK or > notifier_from_errno(0). > > But yes, its a bit of a shit interface. It looks like it was rarely used in earlier kernels as some sort of error detection for the notifier calls, e.g.: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-int profile_handoff_task(struct task_struct * task) 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-{ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c- int ret; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c- read_lock(&handoff_lock); 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c- ret = notifier_call_chain(&task_free_notifier, 0, task); 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c- read_unlock(&handoff_lock); 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c: return (ret == NOTIFY_OK) ? 1 : 0; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-} So NOTIFY_OK was used to state there is no error, while NOTIFY_DONE says the notifier was executed and there might have been errors. The caller may distinguish the results then. -Robert