Re: [PATCH] bdi: Fix another oops in wb_workfn()

From: Tetsuo Handa
Date: Wed Jun 13 2018 - 17:05:09 EST

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.