linux-kernel - Re: [PATCH V2] PCI/DOE: Detect on stack work items automatically

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Date:   Tue, 22 Nov 2022 18:14:06 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "'ira.weiny@...el.com'" <ira.weiny@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Gregory Price <gregory.price@...verge.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V2] PCI/DOE: Detect on stack work items automatically

Now with Thomas added to cc for real.

On Tue, Nov 22, 2022 at 06:13:09PM +0100, Lukas Wunner wrote:
> [+cc Thomas Gleixner, author of dc186ad741c1]
> 
> On Fri, Nov 18, 2022 at 09:20:38AM +0000, David Laight wrote:
> > > From: ira.weiny@...el.com
> > > Sent: 18 November 2022 00:05
> > > 
> > > Work item initialization needs to be done with either
> > > INIT_WORK_ONSTACK() or INIT_WORK() depending on how the work item is
> > > allocated.
> > > 
> > > The callers of pci_doe_submit_task() allocate struct pci_doe_task on the
> > > stack and pci_doe_submit_task() incorrectly used INIT_WORK().
> > > 
> > > Jonathan suggested creating doe task allocation macros such as
> > > DECLARE_CDAT_DOE_TASK_ONSTACK().[1]  The issue with this is the work
> > > function is not known to the callers and must be initialized correctly.
> > > 
> > > A follow up suggestion was to have an internal 'pci_doe_work' item
> > > allocated by pci_doe_submit_task().[2]  This requires an allocation which
> > > could restrict the context where tasks are used.
> > > 
> > > Another idea was to have an intermediate step to initialize the task
> > > struct with a new call.[3]  This added a lot of complexity.
> > > 
> > > Lukas pointed out that object_is_on_stack() is available to detect this
> > > automatically.
> > > 
> > > Use object_is_on_stack() to determine the correct init work function to
> > > call.
> > 
> > This is all a bit strange.
> > The 'onstack' flag is needed for the diagnostic check:
> > 	is_on_stack = object_is_on_stack(addr);
> > 	if (is_on_stack == onstack)
> > 		return;
> > 	pr_warn(...);
> > 	WARN_ON(1);
> > 
> > So setting the flag to the location of the buffer just subverts the check.
> > It that is sane there ought to be a proper way to do it.
> 
> If object_is_on_stack() is sufficient to check whether a struct
> is on the stack or not, why doesn't __init_work() use it to
> auto-detect whether to call debug_object_init_on_stack() or
> debug_object_init()?
> 
> Forcing developers to use a specific initializer for something
> that can be auto-detected is akin to treating them like kids
> and telling them "You didn't say the magic word."
> 
> What's the point?
> 
> Thanks,
> 
> Lukas