Re: [PATCH v5] PM: Support aborting sleep during filesystem sync

[Hillf Danton <hdanton@sina.com>]

On Tue, 21 Oct 2025 13:13:39 -0700 Samuel Wu wrote:
> On Fri, Oct 17, 2025 at 5:17 PM Hillf Danton <hdanton@sina.com> wrote:
> > On Fri, 17 Oct 2025 23:39:06 +0000 Samuel Wu wrote:
> > > +/**
> > > + * pm_sleep_fs_sync - Trigger fs_sync with ability to abort
> > > + *
> > > + * Return 0 on successful file system sync, otherwise returns -EBUSY if file
> > > + * system sync was aborted.
> > > + */
> > > +int pm_sleep_fs_sync(void)
> > > +{
> > > +     bool need_pm_sleep_fs_sync_requeue;
> > > +     unsigned long flags;
> > > +
> > > +     do {
> > > +             spin_lock_irqsave(&pm_sleep_fs_sync_lock, flags);
> > > +             reinit_completion(&pm_sleep_fs_sync_complete);
> >
> > Given difficulty following up here, can you specify why reinit is needed?
> 
> There are two possibilities that make reinit_completion() necessary:
> 1. Suspend abort triggers completion, but is canceled before
> pm_wakeup_pending(), so need reinit to restart the
> wait_for_completion() process.
> 2. Handling back-to-back suspend attempts: after a subsequent suspend
> attempt finishes waiting for a previous suspend's fs_sync to finish,
> we need the reinit to start the wait_for_completion() process of the
> subsequent suspend's fs_sync.
> 
If 1. and 2. matches the comment for wait_for_completion() below,

	static DECLARE_COMPLETION(foo);

	waiter		waker1		waker2
	---		---		---
	for (;;) {
	  reinit_completion(&foo)
	  do anything
	  wait_for_completion(&foo)
			do bar1 	do bar2
			complete(&foo) 	complete(&foo)
	  if (end)
		break;
	}

the chance for reinit to drop one wakeup is not zero.
If drop makes sense, for what do you wait after receiving two wakeups? 

> > > +             /*
> > > +              * Handle the case where a sleep immediately follows a previous
> > > +              * sleep that was aborted during fs_sync. In this case, wait for
> > > +              * the previous filesystem sync to finish. Then do another
> > > +              * filesystem sync so any subsequent filesystem changes are
> > > +              * synced before sleeping.
> > > +              */
> > > +             if (pm_sleep_fs_sync_queued) {
> > > +                     need_pm_sleep_fs_sync_requeue = true;
> > > +             } else {
> > > +                     need_pm_sleep_fs_sync_requeue = false;
> > > +                     pm_sleep_fs_sync_queued = true;
> > > +                     schedule_work(&sync_filesystems);
> > > +             }
> > > +             spin_unlock_irqrestore(&pm_sleep_fs_sync_lock, flags);
> > > +
> > > +             /*
> > > +              * Completion is triggered by fs_sync finishing or an abort sleep
> > > +              * signal, whichever comes first
> > > +              */
> > > +             wait_for_completion(&pm_sleep_fs_sync_complete);
> > > +             if (pm_wakeup_pending())
> > > +                     return -EBUSY;
> > > +     } while (need_pm_sleep_fs_sync_requeue);
> > > +
> > > +     return 0;
> > > +}