linux-kernel - Re: [PATCH] PCI/doe: Fix work struct declaration

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Date:   Wed, 16 Nov 2022 12:20:37 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     ira.weiny@...el.com, Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Gregory Price <gregory.price@...verge.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>, linux-cxl@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH] PCI/doe: Fix work struct declaration

On Wed, Nov 16, 2022 at 11:09:39AM +0100, Lukas Wunner wrote:
> On Mon, Nov 14, 2022 at 05:19:43PM -0800, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > The callers of pci_doe_submit_task() allocate the pci_doe_task on the
> > stack.  This causes the work structure to be allocated on the stack
> > without pci_doe_submit_task() knowing.  Work item initialization needs
> > to be done with either INIT_WORK_ONSTACK() or INIT_WORK() depending on
> > how the work item is allocated.
> > 
> > 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.
> > 
> > Compromise with an intermediate step to initialize the task struct with
> > a new call pci_doe_init_task() which must be called prior to submit
> > task.
> > 
> > [1] https://lore.kernel.org/linux-cxl/20221014151045.24781-1-Jonathan.Cameron@huawei.com/T/#m88a7f50dcce52f30c8bf5c3dcc06fa9843b54a2d
> > [2] https://lore.kernel.org/linux-cxl/20221014151045.24781-1-Jonathan.Cameron@huawei.com/T/#m63c636c5135f304480370924f4d03c00357be667
> 
> We have object_is_on_stack(), included from <linux/sched/task_stack.h>.
> 
> So you could just autosense in pci_doe_submit_task() whether
> pci_doe_task is on the stack and call the appropriate INIT_WORK
> variant.

Nifty, I had no idea object_is_on_stack() existed, thank you!

I wonder if there's an opportunity to use object_is_on_stack()
somewhere in the INIT_WORK() path to find usage mistakes.

Adding it in pci_doe_submit_task() would add some complexity, so I'm
not sure whether it's worth adding it unless we actually have uses for
both cases.

Bjorn