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.
