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

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Message-ID: <Y35a2nhgOUPhhLeS@iweiny-desk3>
Date:   Wed, 23 Nov 2022 09:39:38 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Lukas Wunner <lukas@...ner.de>
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 08:53:16PM +0100, Lukas Wunner wrote:
> 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?

I guess I was not explicit enough in the commit message.  1) it is not used
today.  More importantly 2) it does not work.

Because we are not using it currently I thought it best to remove it rather
than try and fix it.  Once we have a use then we can figure out how to make
sure the doe_mb[*] is valid until all tasks are flushed.

Ira

[*] Probably with a reference on the struct as was discussed before.

	-- https://lore.kernel.org/all/20221122094627.00003f2c@Huawei.com/

> 
> Thanks,
> 
> Lukas