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Date:   Fri, 18 Nov 2022 21:11:58 -0800
From:   Dan Williams <dan.j.williams@intel.com>
To:     "Li, Ming" <ming4.li@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        David Laight <David.Laight@aculab.com>
CC:     Bjorn Helgaas <bhelgaas@google.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Gregory Price <gregory.price@memverge.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Lukas Wunner <lukas@wunner.de>,
        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
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Li, Ming wrote:
> On 11/19/2022 3:46 AM, Dan Williams wrote:
> > Ira Weiny wrote:
> >> 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.
> >>
> >> Ok this brings me back to my previous point and suggested patch.[*]  The
> >> fundamental bug is that the work item is allocated in different code from
> >> the code which uses it.  Separating the work item from the task.
> >>
> >> [*] https://lore.kernel.org/linux-cxl/20221014151045.24781-1-Jonathan.Cameron@huawei.com/T/#m63c636c5135f304480370924f4d03c00357be667
> >>
> >> Bjorn would this solution be acceptable and just use GFP_KERNEL and mark the
> >> required context for pci_doe_submit_task()?
> > 
> > It is a waste to have an allocation when one is not needed. The value of
> > having INIT_WORK_ONSTACK and DECLARE_COMPLETION_ONSTACK is to be clear
> > at the call site that an async context cares about this stack frame not
> > going out of scope.
> > 
> > However, coming full circle, we have zero async users today, and having
> > the completion and work struct in the task are causing a maintenance
> > burden. So let's just rip it out for now with something like the
> > following and circle back to add async support later when it becomes
> > necessary: (only compile tested)
> > 
> > 
> > diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> > index 0dbbe8d39b07..69873cdcc911 100644
> > --- a/drivers/cxl/core/pci.c
> > +++ b/drivers/cxl/core/pci.c
> > @@ -488,21 +488,14 @@ static struct pci_doe_mb *find_cdat_doe(struct device *uport)
> >  		    CXL_DOE_TABLE_ACCESS_TABLE_TYPE_CDATA) |		\
> >  	 FIELD_PREP(CXL_DOE_TABLE_ACCESS_ENTRY_HANDLE, (entry_handle)))
> >  
> > -static void cxl_doe_task_complete(struct pci_doe_task *task)
> > -{
> > -	complete(task->private);
> > -}
> > -
> >  struct cdat_doe_task {
> >  	u32 request_pl;
> >  	u32 response_pl[32];
> > -	struct completion c;
> >  	struct pci_doe_task task;
> >  };
> >  
> >  #define DECLARE_CDAT_DOE_TASK(req, cdt)                       \
> >  struct cdat_doe_task cdt = {                                  \
> > -	.c = COMPLETION_INITIALIZER_ONSTACK(cdt.c),           \
> >  	.request_pl = req,				      \
> >  	.task = {                                             \
> >  		.prot.vid = PCI_DVSEC_VENDOR_ID_CXL,        \
> > @@ -511,8 +504,6 @@ struct cdat_doe_task cdt = {                                  \
> >  		.request_pl_sz = sizeof(cdt.request_pl),      \
> >  		.response_pl = cdt.response_pl,               \
> >  		.response_pl_sz = sizeof(cdt.response_pl),    \
> > -		.complete = cxl_doe_task_complete,            \
> > -		.private = &cdt.c,                            \
> >  	}                                                     \
> >  }
> >  
> > @@ -523,12 +514,12 @@ static int cxl_cdat_get_length(struct device *dev,
> >  	DECLARE_CDAT_DOE_TASK(CDAT_DOE_REQ(0), t);
> >  	int rc;
> >  
> > -	rc = pci_doe_submit_task(cdat_doe, &t.task);
> > +	rc = pci_doe_submit_task_wait(cdat_doe, &t.task);
> >  	if (rc < 0) {
> >  		dev_err(dev, "DOE submit failed: %d", rc);
> >  		return rc;
> >  	}
> > -	wait_for_completion(&t.c);
> > +
> >  	if (t.task.rv < sizeof(u32))
> >  		return -EIO;
> >  
> > @@ -552,12 +543,11 @@ static int cxl_cdat_read_table(struct device *dev,
> >  		u32 *entry;
> >  		int rc;
> >  
> > -		rc = pci_doe_submit_task(cdat_doe, &t.task);
> > +		rc = pci_doe_submit_task_wait(cdat_doe, &t.task);
> >  		if (rc < 0) {
> >  			dev_err(dev, "DOE submit failed: %d", rc);
> >  			return rc;
> >  		}
> > -		wait_for_completion(&t.c);
> >  		/* 1 DW header + 1 DW data min */
> >  		if (t.task.rv < (2 * sizeof(u32)))
> >  			return -EIO;
> > diff --git a/drivers/pci/doe.c b/drivers/pci/doe.c
> > index e402f05068a5..115a8ff14afc 100644
> > --- a/drivers/pci/doe.c
> > +++ b/drivers/pci/doe.c
> > @@ -18,7 +18,6 @@
> >  #include <linux/mutex.h>
> >  #include <linux/pci.h>
> >  #include <linux/pci-doe.h>
> > -#include <linux/workqueue.h>
> >  
> >  #define PCI_DOE_PROTOCOL_DISCOVERY 0
> >  
> > @@ -40,7 +39,6 @@
> >   * @cap_offset: Capability offset
> >   * @prots: Array of protocols supported (encoded as long values)
> >   * @wq: Wait queue for work item
> > - * @work_queue: Queue of pci_doe_work items
> >   * @flags: Bit array of PCI_DOE_FLAG_* flags
> >   */
> >  struct pci_doe_mb {
> > @@ -49,7 +47,6 @@ struct pci_doe_mb {
> >  	struct xarray prots;
> >  
> >  	wait_queue_head_t wq;
> > -	struct workqueue_struct *work_queue;
> >  	unsigned long flags;
> >  };
> >  
> > @@ -211,7 +208,6 @@ static int pci_doe_recv_resp(struct pci_doe_mb *doe_mb, struct pci_doe_task *tas
> >  static void signal_task_complete(struct pci_doe_task *task, int rv)
> >  {
> >  	task->rv = rv;
> > -	task->complete(task);
> >  }
> >  
> >  static void signal_task_abort(struct pci_doe_task *task, int rv)
> > @@ -231,10 +227,9 @@ static void signal_task_abort(struct pci_doe_task *task, int rv)
> >  	signal_task_complete(task, rv);
> >  }
> >  
> > -static void doe_statemachine_work(struct work_struct *work)
> > +
> > +static void exec_task(struct pci_doe_task *task)
> >  {
> > -	struct pci_doe_task *task = container_of(work, struct pci_doe_task,
> > -						 work);
> >  	struct pci_doe_mb *doe_mb = task->doe_mb;
> >  	struct pci_dev *pdev = doe_mb->pdev;
> >  	int offset = doe_mb->cap_offset;
> > @@ -295,18 +290,12 @@ static void doe_statemachine_work(struct work_struct *work)
> >  	signal_task_complete(task, rc);
> >  }
> >  
> > -static void pci_doe_task_complete(struct pci_doe_task *task)
> > -{
> > -	complete(task->private);
> > -}
> > -
> >  static int pci_doe_discovery(struct pci_doe_mb *doe_mb, u8 *index, u16 *vid,
> >  			     u8 *protocol)
> >  {
> >  	u32 request_pl = FIELD_PREP(PCI_DOE_DATA_OBJECT_DISC_REQ_3_INDEX,
> >  				    *index);
> >  	u32 response_pl;
> > -	DECLARE_COMPLETION_ONSTACK(c);
> >  	struct pci_doe_task task = {
> >  		.prot.vid = PCI_VENDOR_ID_PCI_SIG,
> >  		.prot.type = PCI_DOE_PROTOCOL_DISCOVERY,
> > @@ -314,17 +303,13 @@ static int pci_doe_discovery(struct pci_doe_mb *doe_mb, u8 *index, u16 *vid,
> >  		.request_pl_sz = sizeof(request_pl),
> >  		.response_pl = &response_pl,
> >  		.response_pl_sz = sizeof(response_pl),
> > -		.complete = pci_doe_task_complete,
> > -		.private = &c,
> >  	};
> >  	int rc;
> >  
> > -	rc = pci_doe_submit_task(doe_mb, &task);
> > +	rc = pci_doe_submit_task_wait(doe_mb, &task);
> >  	if (rc < 0)
> >  		return rc;
> >  
> > -	wait_for_completion(&c);
> > -
> >  	if (task.rv != sizeof(response_pl))
> >  		return -EIO;
> >  
> > @@ -376,13 +361,6 @@ static void pci_doe_xa_destroy(void *mb)
> >  	xa_destroy(&doe_mb->prots);
> >  }
> >  
> > -static void pci_doe_destroy_workqueue(void *mb)
> > -{
> > -	struct pci_doe_mb *doe_mb = mb;
> > -
> > -	destroy_workqueue(doe_mb->work_queue);
> > -}
> > -
> >  static void pci_doe_flush_mb(void *mb)
> >  {
> >  	struct pci_doe_mb *doe_mb = mb;
> > @@ -393,9 +371,6 @@ static void pci_doe_flush_mb(void *mb)
> >  	/* Cancel an in progress work item, if necessary */
> >  	set_bit(PCI_DOE_FLAG_CANCEL, &doe_mb->flags);
> >  	wake_up(&doe_mb->wq);
> > -
> > -	/* Flush all work items */
> > -	flush_workqueue(doe_mb->work_queue);
> >  }
> >  
> >  /**
> > @@ -429,19 +404,6 @@ struct pci_doe_mb *pcim_doe_create_mb(struct pci_dev *pdev, u16 cap_offset)
> >  	if (rc)
> >  		return ERR_PTR(rc);
> >  
> > -	doe_mb->work_queue = alloc_ordered_workqueue("%s %s DOE [%x]", 0,
> > -						dev_driver_string(&pdev->dev),
> > -						pci_name(pdev),
> > -						doe_mb->cap_offset);
> > -	if (!doe_mb->work_queue) {
> > -		pci_err(pdev, "[%x] failed to allocate work queue\n",
> > -			doe_mb->cap_offset);
> > -		return ERR_PTR(-ENOMEM);
> > -	}
> > -	rc = devm_add_action_or_reset(dev, pci_doe_destroy_workqueue, doe_mb);
> > -	if (rc)
> > -		return ERR_PTR(rc);
> > -
> >  	/* Reset the mailbox by issuing an abort */
> >  	rc = pci_doe_abort(doe_mb);
> >  	if (rc) {
> > @@ -496,23 +458,20 @@ bool pci_doe_supports_prot(struct pci_doe_mb *doe_mb, u16 vid, u8 type)
> >  EXPORT_SYMBOL_GPL(pci_doe_supports_prot);
> >  
> >  /**
> > - * pci_doe_submit_task() - Submit a task to be processed by the state machine
> > + * pci_doe_submit_task_wait() - Submit and execute a task
> >   *
> >   * @doe_mb: DOE mailbox capability to submit to
> > - * @task: task to be queued
> > + * @task: task to be run
> >   *
> > - * Submit a DOE task (request/response) to the DOE mailbox to be processed.
> > - * Returns upon queueing the task object.  If the queue is full this function
> > - * will sleep until there is room in the queue.
> > - *
> > - * task->complete will be called when the state machine is done processing this
> > - * task.
> > + * Submit and run DOE task (request/response) to the DOE mailbox to be
> > + * processed.
> >   *
> >   * Excess data will be discarded.
> >   *
> > - * RETURNS: 0 when task has been successfully queued, -ERRNO on error
> > + * RETURNS: 0 when task was executed, the @task->rv holds the status
> > + * result of the executed opertion, -ERRNO on failure to submit.
> >   */
> > -int pci_doe_submit_task(struct pci_doe_mb *doe_mb, struct pci_doe_task *task)
> > +int pci_doe_submit_task_wait(struct pci_doe_mb *doe_mb, struct pci_doe_task *task)
> >  {
> >  	if (!pci_doe_supports_prot(doe_mb, task->prot.vid, task->prot.type))
> >  		return -EINVAL;
> > @@ -529,8 +488,8 @@ int pci_doe_submit_task(struct pci_doe_mb *doe_mb, struct pci_doe_task *task)
> >  		return -EIO;
> >  
> >  	task->doe_mb = doe_mb;
> > -	INIT_WORK(&task->work, doe_statemachine_work);
> > -	queue_work(doe_mb->work_queue, &task->work);
> > +	exec_task(task);
> > +
> >  	return 0;
> >  }
> > -EXPORT_SYMBOL_GPL(pci_doe_submit_task);
> > +EXPORT_SYMBOL_GPL(pci_doe_submit_task_wait);
> > diff --git a/include/linux/pci-doe.h b/include/linux/pci-doe.h
> > index ed9b4df792b8..c94122a66221 100644
> > --- a/include/linux/pci-doe.h
> > +++ b/include/linux/pci-doe.h
> > @@ -30,8 +30,6 @@ struct pci_doe_mb;
> >   * @response_pl_sz: Size of the response payload (bytes)
> >   * @rv: Return value.  Length of received response or error (bytes)
> >   * @complete: Called when task is complete
> > - * @private: Private data for the consumer
> > - * @work: Used internally by the mailbox
> >   * @doe_mb: Used internally by the mailbox
> >   *
> >   * The payload sizes and rv are specified in bytes with the following
> > @@ -50,11 +48,6 @@ struct pci_doe_task {
> >  	u32 *response_pl;
> >  	size_t response_pl_sz;
> >  	int rv;
> > -	void (*complete)(struct pci_doe_task *task);
> > -	void *private;
> > -
> > -	/* No need for the user to initialize these fields */
> > -	struct work_struct work;
> >  	struct pci_doe_mb *doe_mb;
> >  };
> >  
> > @@ -72,6 +65,5 @@ struct pci_doe_task {
> >  
> >  struct pci_doe_mb *pcim_doe_create_mb(struct pci_dev *pdev, u16 cap_offset);
> >  bool pci_doe_supports_prot(struct pci_doe_mb *doe_mb, u16 vid, u8 type);
> > -int pci_doe_submit_task(struct pci_doe_mb *doe_mb, struct pci_doe_task *task);
> > -
> > +int pci_doe_submit_task_wait(struct pci_doe_mb *doe_mb, struct pci_doe_task *task);
> >  #endif
> > 
> > 
> 
> good to see that we can have a sync interface.
> I think that we still need some methods to garantee doe_task can be
> handled one by one in doe_mb?  When more than one kernel thread are
> going to transfer data over a same doe_mb, only one kernel thread can
> use it and others will failed in exec_task().
> 

Oh, good catch, yes, this likely needs a mutex_lock_interruptible() over
exec_task(), or similar.