On Thu 14-06-18 06:04:04, Tetsuo Handa wrote: > On 2018/06/14 1:45, Jan Kara wrote: > > On Wed 13-06-18 09:25:03, Linus Torvalds wrote: > >> On Wed, Jun 13, 2018 at 9:21 AM Tetsuo Handa > >> <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote: > >>> > >>> Since multiple addresses share bit_wait_table[256], isn't it possible that > >>> cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by > >>> hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from > >>> wb_shutdown())? I think that we cannot be sure without confirming that > >>> test_bit(WB_shutting_down, &wb->state) == false after returning from schedule(). > >> > >> Right. > >> > >> That's _always_ true, btw. Something else entirely could have woken > >> you up. TASK_UNINTERRUPTIBLE does not mean "nothing else wakes me", it > >> just means "_signals_ don't wake me". > >> > >> So every single sleep always needs to be in a loop. Always. > > > > Agreed and in my patch it actually is in a loop - the one iterating the > > list of active writeback structures. If we get a false wakeup, we find the > > same structure in the list again and wait again... > > Indeed. I overlooked that wb = list_first_entry() will select same wb again > if cgwb_remove_from_bdi_list() is not yet called. Well, we could update > "(in which case we also wait for it to finish)" part or move the body of > cgwb_start_shutdown() to cgwb_bdi_unregister() so that it becomes clear > that false wake-up is not a problem in this case. I prefer to keep the wb shutdown in a separate function but I've added some comments to explain that. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
