On Thu, Apr 28, 2016 at 10:30:55PM +0800, Chao Yu wrote: > On 2016/4/28 22:03, Peter Zijlstra wrote: > > On Thu, Apr 28, 2016 at 07:51:04PM +0800, Chao Yu wrote: > >>> On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote: > >>>> From: Chao Yu <yuch...@huawei.com> > >>>> > >>>> The following condition can happen in a preemptible kernel, it may cause > >>>> checkpointer hunging. > >>>> > >>>> CPU0: CPU1: > >>>> - write_checkpoint > >>>> - do_checkpoint > >>>> - wait_on_all_pages_writeback > >>>> - f2fs_write_end_io > >>>> - wake_up > >>>> this is last writebacked page, but > >>>> no sleeper in sbi->cp_wait wait > >>>> queue, wake_up is not been called. > >>>> - prepare_to_wait(TASK_UNINTERRUPTIBLE) > >>>> Here, current task can been preempted, > >>>> but there will be no waker since last > >>>> write_end_io has bypassed wake_up. So > >>>> current task will sleep forever. > > > > But here, you should be verifying if you really should go sleep; as the > > code did; it tests for !get_pages(, F2FS_WRITEBACK), and if you've just > > completed that very last one, this will break out. > > You mean after being preempted with TASK_UNINTERRUPTIBLE status, that task > still > has chance to be scheduled to check '!get_pages(, F2FS_WRITEBACK)', is that > right?
Yes, preemption ignores task_struct::state.
