linux-kernel - Re: [PATCH v3] PM: suspend: Add sysfs attribute to control the "sync on suspend" behavior

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Date:   Thu, 30 Jan 2020 13:19:26 +0100
From:   Pavel Machek <pavel@....cz>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Jonas Meurer <jonas@...esources.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Len Brown <len.brown@...el.com>,
        Tim Dittler <tim.dittler@...temli.org>,
        Yannik Sembritzki <yannik@...britzki.me>
Subject: Re: [PATCH v3] PM: suspend: Add sysfs attribute to control the "sync
 on suspend" behavior

Hi!

> > The sysfs attribute `/sys/power/sync_on_suspend` controls, whether or not
> > filesystems are synced by the kernel before system suspend.
> >
> > Congruously, the behaviour of build-time switch CONFIG_SUSPEND_SKIP_SYNC
> > is slightly changed: It now defines the run-tim default for the new sysfs
> > attribute `/sys/power/sync_on_suspend`.
> >
> > The run-time attribute is added because the existing corresponding
> > build-time Kconfig flag for (`CONFIG_SUSPEND_SKIP_SYNC`) is not flexible
> > enough. E.g. Linux distributions that provide pre-compiled kernels
> > usually want to stick with the default (sync filesystems before suspend)
> > but under special conditions this needs to be changed.
> >
> > One example for such a special condition is user-space handling of
> > suspending block devices (e.g. using `cryptsetup luksSuspend` or `dmsetup
> > suspend`) before system suspend. The Kernel trying to sync filesystems
> > after the underlying block device already got suspended obviously leads
> > to dead-locks. Be aware that you have to take care of the filesystem sync
> > yourself before suspending the system in those scenarios.
> >
> > Signed-off-by: Jonas Meurer <jonas@...esources.org>
> 
> Applied as 5.6 material with minor changes in the ABI document, thanks!

For the record, I still believe this is bad idea.

User should not have to tweak variables in /sys for system not to
deadlock with cryptsetup.. and we are stuck with this pretty much
forever.

									Pavel