linux-kernel - Re: [PATCH v10 0/5] shut down devices asynchronously

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Message-ID: <aGgAuje4tpIOveFc@sultan-box>
Date: Fri, 4 Jul 2025 09:26:34 -0700
From: Sultan Alsawaf <sultan@...neltoast.com>
To: David Jeffery <djeffery@...hat.com>
Cc: Jeremy Allison <jra@...ba.org>, Christoph Hellwig <hch@....de>,
	Stuart Hayes <stuart.w.hayes@...il.com>,
	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Martin Belanger <Martin.Belanger@...l.com>,
	Oliver O'Halloran <oohall@...il.com>,
	Daniel Wagner <dwagner@...e.de>, Keith Busch <kbusch@...nel.org>,
	Lukas Wunner <lukas@...ner.de>, Jeremy Allison <jallison@....com>,
	Jens Axboe <axboe@...com>, Sagi Grimberg <sagi@...mberg.me>,
	linux-nvme@...ts.infradead.org,
	Nathan Chancellor <nathan@...nel.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Bert Karwatzki <spasswolf@....de>
Subject: Re: [PATCH v10 0/5] shut down devices asynchronously

On Fri, Jul 04, 2025 at 09:45:44AM -0400, David Jeffery wrote:
> On Thu, Jul 3, 2025 at 12:13 PM Jeremy Allison <jra@...ba.org> wrote:
> >
> > On Thu, Jul 03, 2025 at 01:46:56PM +0200, Christoph Hellwig wrote:
> > >On Wed, Jun 25, 2025 at 03:18:48PM -0500, Stuart Hayes wrote:
> > >> Address resource and timing issues when spawning a unique async thread
> > >> for every device during shutdown:
> > >>   * Make the asynchronous threads able to shut down multiple devices,
> > >>     instead of spawning a unique thread for every device.
> > >>   * Modify core kernel async code with a custom wake function so it
> > >>     doesn't wake up threads waiting to synchronize every time the cookie
> > >>     changes
> > >
> > >Given all these thread spawning issues, why can't we just go back
> > >to the approach that kicks off shutdown asynchronously and then waits
> > >for it without spawning all these threads?
> >
> > It isn't just an nvme issue. Red Hat found the same issue
> > with SCSI devices.
> >
> > My colleague Sultan Alsawaf posted a simpler fix for the
> > earlier patch here:
> >
> > https://lists.infradead.org/pipermail/linux-nvme/2025-January/053666.html
> >
> > Maybe this could be explored.
> >
> 
> Unfortunately, this approach looks flawed. If I am reading it right,
> it assumes async shutdown devices do not have dependencies on sync
> shutdown devices.

It does not make any such assumption. Dependency on a sync device is handled
through a combination of queue_device_async_shutdown() setting an async device's
shutdown_after and the synchronous shutdown loop dispatching an "async" shutdown
for a sync device when it encounters a sync device that has a downstream async
dependency.

> Maintaining all the dependencies is the core problem and source of the
> complexity of the async shutdown patches.

I am acutely aware. Please take a closer look at my patch.

Sultan