linux-kernel - Re: [PATCH V2 1/2] PCI/DOE: Remove the pci_doe_flush

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Date:   Tue, 22 Nov 2022 20:53:16 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     ira.weiny@...el.com
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Gregory Price <gregory.price@...verge.com>,
        "Li, Ming" <ming4.li@...el.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org
Subject: Re: [PATCH V2 1/2] PCI/DOE: Remove the pci_doe_flush_mb() call

On Tue, Nov 22, 2022 at 07:53:23AM -0800, ira.weiny@...el.com wrote:
> Each struct doe_mb is managed as part of the PCI device.  They can't go
> away as long as the PCI device exists.  pci_doe_flush_mb() was set up to
> flush the workqueue and prevent any further submissions to the mailboxes
> when the PCI device goes away.  Unfortunately, this was fundamentally
> flawed.  There was no guarantee that a struct doe_mb remained after
> pci_doe_flush_mb() returned.  Therefore, the doe_mb state could be
> invalid when those threads waiting on the workqueue were flushed.
> 
> Fortunately the current code is safe because all callers make a
> synchronous call to pci_doe_submit_task() and maintain a reference on the
> PCI device.
> 
> For these reasons, pci_doe_flush_mb() will never be called while tasks
> are being processed and there is no use for it.

Going forward my plan is to allocate all existing DOE mailboxes
of a device upon enumeration.  That will allow concurrent use
of a mailbox by multiple drivers.

When a pci_dev goes away, say, because it's been hot-removed,
we need a way to abort all ongoing DOE exchanges.

pci_doe_flush_mb() seems to do just that so I'm not sure why
it's being removed?

Thanks,

Lukas