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

[Dan Williams]

Lukas Wunner wrote:
> 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@xxxxxxxxx
> > > > 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?

I had this initial reaction as well, but INIT_WORK_ONSTACK() documents
an important detail of the object's lifetime. Here are 2 examples of
functions that would become trickier to read if the kernel did a
global s/INIT_WORK_ONSTACK()/INIT_WORK()/

    synchronize_rcu_expedited_queue_work()
    insert_wq_barrier()

...where those take arguments that are known to come from the stack and
be used in async context.