linux-kernel - Re: [PATCH RFC v5 1/5] pci: report surprise removal event

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Message-ID: <20250718044006-mutt-send-email-mst@kernel.org>
Date: Fri, 18 Jul 2025 04:40:22 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Lukas Wunner <lukas@...ner.de>
Cc: linux-kernel@...r.kernel.org, Keith Busch <kbusch@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Parav Pandit <parav@...dia.com>, virtualization@...ts.linux.dev,
	stefanha@...hat.com, alok.a.tiwari@...cle.com,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH RFC v5 1/5] pci: report surprise removal event

On Fri, Jul 18, 2025 at 06:35:56AM +0200, Lukas Wunner wrote:
> On Thu, Jul 17, 2025 at 07:31:57PM -0400, Michael S. Tsirkin wrote:
> > On Thu, Jul 17, 2025 at 10:12:03PM +0200, Lukas Wunner wrote:
> > > pciehp_handle_presence_or_link_change() is called from pciehp_ist(),
> > > the IRQ thread.  During safe removal the IRQ thread is busy in
> > > pciehp_unconfigure_device() and waiting for the driver to unbind
> > > from devices being safe-removed.
> > 
> > Confused. I thought safe removal happens in the userspace thread
> > that wrote into sysfs?
> 
> No, the userspace thread synthesizes a DISABLE_SLOT event,
> calls irq_wake_thread(), then waits for the IRQ thread to
> finish handling that event.  See pciehp_sysfs_disable_slot().
> 
> Until 2018 we indeed brought down the slot in the userspace
> thread, but that required locking between the workqueue fed
> by the interrupt handler on the one hand and the userspace
> thread on the other hand.  It was difficult to reason about
> the code.
> 
> We had bug reports about slots flapping the link or presence
> bits on slot bringdown that we could easily address by handling
> everything in the IRQ thread, see 3943af9d01e9.  The same was
> reported for slot bringup and addressed by 6c35a1ac3da6.
> 
> This wouldn't have been possible with the architecture prior
> to 2018, at least not this easily.
> 
> Thanks,
> 
> Lukas

Got it, thanks!

-- 
MST