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

From: Jan Kara
Date: Wed Jun 13 2018 - 12:45:17 EST

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...

Jan Kara <jack@xxxxxxxx>