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

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Message-ID: <CAG2KctoJ+1x61KNmDj_52J1_Y3vyox7UNceFw6_WtbRMA_1vYA@mail.gmail.com>
Date: Wed, 22 Oct 2025 11:41:37 -0700
From: Samuel Wu <wusamuel@...gle.com>
To: Hillf Danton <hdanton@...a.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Pavel Machek <pavel@...nel.org>, Len Brown <lenb@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Danilo Krummrich <dakr@...nel.org>, 
	Saravana Kannan <saravanak@...gle.com>, kernel-team@...roid.com, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] PM: Support aborting sleep during filesystem sync

On Tue, Oct 21, 2025 at 6:16 PM Hillf Danton <hdanton@...a.com> wrote:
>
> On Tue, 21 Oct 2025 13:13:39 -0700 Samuel Wu wrote:
> > On Fri, Oct 17, 2025 at 5:17 PM Hillf Danton <hdanton@...a.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?
>

If I understand correctly, you are referring to the case where
multiple wakers trigger wait_for_complete() simultaneously, hence
having at least one waker's complete() being ignored?

If so, I see two possibilities with multiple wakers:
1. fs_sync finishing + suspend abort1 + ... + suspend abortN
2. suspend abort1 + ... + suspend abortN

Simplifying, if two wakers come in simultaneously, while one of the
wakers may have its complete() ignored, the state of that waker is
still checked after wait_for_completion(), with
if(pm_wakeup_pending()) and while(need_pm_sleep_fs_sync_requeue) for
suspend aborts and fs_sync finishing respectively.

> > > > +             /*
> > > > +              * 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;
> > > > +}
>
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