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Date: Wed, 16 Nov 2022 12:57:55 -0800
From: Ira Weiny <ira.weiny@...el.com>
To: Bjorn Helgaas <helgaas@...nel.org>
CC: Lukas Wunner <lukas@...ner.de>,
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 12:20:37PM -0600, Bjorn Helgaas wrote:
> 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!
Indeed! Neither did I! thanks!
>
> I wonder if there's an opportunity to use object_is_on_stack()
> somewhere in the INIT_WORK() path to find usage mistakes.
I'm thinking we could make INIT_WORK do the right thing all the time. Not sure
what the overhead of object_is_on_stack() is.
>
> 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.
I think if we don't do something we have to document that
pci_doe_submit_task() only works with tasks on the stack.
I would rather just make pci_doe_submit_task() correct and not complicate the
callers. object_is_on_stack() can't be enough overhead to be worried about in
this call path can it?
Actually after writing all that I wonder if we can't push the use of
object_is_on_stack() into the debug code? Something like below (completely
untested)? I think this could be pushed even further down but I'd like to get
opinions before attempting a change which will have a wider blast radius.
Ira
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index a0143dd24430..4cc50b554a29 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -199,7 +199,7 @@ struct execute_work {
struct delayed_work n = __DELAYED_WORK_INITIALIZER(n, f, TIMER_DEFERRABLE)
#ifdef CONFIG_DEBUG_OBJECTS_WORK
-extern void __init_work(struct work_struct *work, int onstack);
+extern void __init_work(struct work_struct *work);
extern void destroy_work_on_stack(struct work_struct *work);
extern void destroy_delayed_work_on_stack(struct delayed_work *work);
static inline unsigned int work_static(struct work_struct *work)
@@ -207,7 +207,7 @@ static inline unsigned int work_static(struct work_struct *work)
return *work_data_bits(work) & WORK_STRUCT_STATIC;
}
#else
-static inline void __init_work(struct work_struct *work, int onstack) { }
+static inline void __init_work(struct work_struct *work) { }
static inline void destroy_work_on_stack(struct work_struct *work) { }
static inline void destroy_delayed_work_on_stack(struct delayed_work *work) { }
static inline unsigned int work_static(struct work_struct *work) { return 0; }
@@ -221,20 +221,20 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }
* to generate better code.
*/
#ifdef CONFIG_LOCKDEP
-#define __INIT_WORK(_work, _func, _onstack) \
+#define __INIT_WORK(_work, _func) \
do { \
static struct lock_class_key __key; \
\
- __init_work((_work), _onstack); \
+ __init_work(_work); \
(_work)->data = (atomic_long_t) WORK_DATA_INIT(); \
lockdep_init_map(&(_work)->lockdep_map, "(work_completion)"#_work, &__key, 0); \
INIT_LIST_HEAD(&(_work)->entry); \
(_work)->func = (_func); \
} while (0)
#else
-#define __INIT_WORK(_work, _func, _onstack) \
+#define __INIT_WORK(_work, _func) \
do { \
- __init_work((_work), _onstack); \
+ __init_work(_work); \
(_work)->data = (atomic_long_t) WORK_DATA_INIT(); \
INIT_LIST_HEAD(&(_work)->entry); \
(_work)->func = (_func); \
@@ -242,10 +242,10 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }
#endif
#define INIT_WORK(_work, _func) \
- __INIT_WORK((_work), (_func), 0)
+ __INIT_WORK((_work), (_func))
#define INIT_WORK_ONSTACK(_work, _func) \
- __INIT_WORK((_work), (_func), 1)
+ __INIT_WORK((_work), (_func))
#define __INIT_DELAYED_WORK(_work, _func, _tflags) \
do { \
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 7cd5f5e7e0a1..7d87300cfbc6 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -515,9 +515,9 @@ static inline void debug_work_deactivate(struct work_struct *work)
debug_object_deactivate(work, &work_debug_descr);
}
-void __init_work(struct work_struct *work, int onstack)
+void __init_work(struct work_struct *work)
{
- if (onstack)
+ if (object_is_on_stack(work))
debug_object_init_on_stack(work, &work_debug_descr);
else
debug_object_init(work, &work_debug_descr);
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