linux-kernel - Re: [RFC 5/5] pm: remove kernel thread freezing

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Date:   Wed, 4 Oct 2017 08:04:49 +1100
From:   Dave Chinner <david@...morbit.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     viro@...iv.linux.org.uk, bart.vanassche@....com,
        ming.lei@...hat.com, tytso@....edu, darrick.wong@...cle.com,
        jikos@...nel.org, rjw@...ysocki.net, pavel@....cz,
        len.brown@...el.com, linux-fsdevel@...r.kernel.org,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        todd.e.brandt@...ux.intel.com, nborisov@...e.com, jack@...e.cz,
        martin.petersen@...cle.com, ONeukum@...e.com,
        oleksandr@...alenko.name, oleg.b.antonyan@...il.com,
        linux-pm@...r.kernel.org, linux-block@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 5/5] pm: remove kernel thread freezing

On Tue, Oct 03, 2017 at 11:53:13AM -0700, Luis R. Rodriguez wrote:
> Now that all filesystems which used to rely on kthread
> freezing have been converted to filesystem freeze/thawing
> we can remove the kernel kthread freezer.

Really? There's no other subsystem that relies on kernel thread
and workqueue freezing to function correctly on suspend?

> -/**
> - * freeze_kernel_threads - Make freezable kernel threads go to the refrigerator.
> - *
> - * On success, returns 0.  On failure, -errno and only the kernel threads are
> - * thawed, so as to give a chance to the caller to do additional cleanups
> - * (if any) before thawing the userspace tasks. So, it is the responsibility
> - * of the caller to thaw the userspace tasks, when the time is right.
> - */
> -int freeze_kernel_threads(void)
> -{
> -	int error;
> -
> -	pr_info("Freezing remaining freezable tasks ... ");
> -
> -	pm_nosig_freezing = true;
> -	error = try_to_freeze_tasks(false);

This freezes workqueues as well as kernel threads, so this affects
any subsystem that uses WQ_FREEZABLE. A quick glance tells me this
includes graphics drivers, spi devices, usb hubs, power management,
and a few filesystems, too.

> -	if (!error)
> -		pr_cont("done.");
> -
> -	pr_cont("\n");
> -	BUG_ON(in_atomic());
> -
> -	if (error)
> -		thaw_kernel_threads();
> -	return error;
> -}
> -
>  void thaw_processes(void)
>  {
>  	struct task_struct *g, *p;
> @@ -234,23 +207,3 @@ void thaw_processes(void)
>  	pr_cont("done.\n");
>  	trace_suspend_resume(TPS("thaw_processes"), 0, false);
>  }
> -
> -void thaw_kernel_threads(void)
> -{
> -	struct task_struct *g, *p;
> -
> -	pm_nosig_freezing = false;
> -	pr_info("Restarting kernel threads ... ");
> -
> -	thaw_workqueues();

And this is where the workqueues are thawed.

So I doubt we can safely remove all this code like this...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com