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Date:   Tue, 22 Nov 2022 18:13:09 +0100
From:   Lukas Wunner <lukas@wunner.de>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     "'ira.weiny@intel.com'" <ira.weiny@intel.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Gregory Price <gregory.price@memverge.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-cxl@vger.kernel.org" <linux-cxl@vger.kernel.org>
Subject: Re: [PATCH V2] PCI/DOE: Detect on stack work items automatically
Message-ID: <20221122171309.GA11310@wunner.de>
References: <20221118000524.1477383-1-ira.weiny@intel.com>
 <e59f83f3ca4149d098efe43b48fecd1b@AcuMS.aculab.com>
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[+cc Thomas Gleixner, author of dc186ad741c1]

On Fri, Nov 18, 2022 at 09:20:38AM +0000, David Laight wrote:
> > From: ira.weiny@intel.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