On 2016/4/28 22:37, Peter Zijlstra wrote: > 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 <[email protected]> >>>>>> >>>>>> 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.
Got it. Thanks very much for your help! :) >
