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Message-ID: <637b98358f7d0_1ee2429488@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Mon, 21 Nov 2022 07:24:37 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: <ira.weiny@...el.com>, Dan Williams <dan.j.williams@...el.com>,
"Bjorn Helgaas" <bhelgaas@...gle.com>
CC: Ira Weiny <ira.weiny@...el.com>,
Gregory Price <gregory.price@...verge.com>,
Jonathan Cameron <Jonathan.Cameron@...wei.com>,
"Li, Ming" <ming4.li@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>,
Lukas Wunner <lukas@...ner.de>,
Alison Schofield <alison.schofield@...el.com>,
<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<linux-pci@...r.kernel.org>
Subject: RE: [PATCH] PCI/DOE: Remove asynchronous task support
ira.weiny@ wrote:
> From: Ira Weiny <ira.weiny@...el.com>
>
> Gregory Price and Jonathan Cameron reported a bug within
> pci_doe_submit_task().[1] The issue was that work item initialization
> needs to be done with either INIT_WORK_ONSTACK() or INIT_WORK()
> depending on how the work item is allocated.
>
> Initially, it was anticipated that DOE tasks were going to need to be
> submitted asynchronously and the code was designed thusly. Many
> alternatives were discussed to fix the work initialization issue.[2]
>
> However, all current users submit tasks synchronously and this has
> therefore become an unneeded maintenance burden. Remove the extra
> maintenance burden by replacing asynchronous task submission with
> a synchronous wait function.[3]
>
> [1] https://lore.kernel.org/linux-cxl/20221014151045.24781-1-Jonathan.Cameron@huawei.com/T/#m88a7f50dcce52f30c8bf5c3dcc06fa9843b54a2d
> [2] https://lore.kernel.org/linux-cxl/Y3kSDQDur+IUDs2O@iweiny-mobl/T/#m0f057773d9c75432fcfcc54a2604483fe82abe92
> [3] https://lore.kernel.org/linux-cxl/Y3kSDQDur+IUDs2O@iweiny-mobl/T/#m32d3f9b208ef7486bc148d94a326b26b2d3e69ff
>
> Reported-by: Gregory Price <gregory.price@...verge.com>
> Reported-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> Suggested-by: "Li, Ming" <ming4.li@...el.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
>
> ---
> Thanks to Dan for the bulk of the patch.
> Thanks to Ming for pointing out the need for a lock to prevent more
> than 1 task from being processed at a time.
> ---
> drivers/cxl/core/pci.c | 16 ++------
> drivers/pci/doe.c | 83 ++++++++++++++---------------------------
> include/linux/pci-doe.h | 10 +----
> 3 files changed, 32 insertions(+), 77 deletions(-)
>
> diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> index 9240df53ed87..58977e0712b6 100644
> --- a/drivers/cxl/core/pci.c
> +++ b/drivers/cxl/core/pci.c
> @@ -490,21 +490,14 @@ 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, \
> @@ -513,8 +506,6 @@ 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, \
> } \
> }
>
> @@ -525,12 +516,12 @@ 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;
>
> @@ -554,12 +545,11 @@ 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..41a75112b39b 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 +39,7 @@
> * @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
> + * @exec_lock: Lock to ensure 1 task is processed at a time
> * @flags: Bit array of PCI_DOE_FLAG_* flags
> */
> struct pci_doe_mb {
> @@ -49,7 +48,7 @@ struct pci_doe_mb {
> struct xarray prots;
>
> wait_queue_head_t wq;
> - struct workqueue_struct *work_queue;
> + struct mutex exec_lock;
> unsigned long flags;
> };
>
> @@ -211,7 +210,6 @@ 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 +229,8 @@ 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 +291,12 @@ 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 +304,13 @@ 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 +362,6 @@ 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;
> @@ -390,12 +369,9 @@ static void pci_doe_flush_mb(void *mb)
> /* Stop all pending work items from starting */
> set_bit(PCI_DOE_FLAG_DEAD, &doe_mb->flags);
>
> - /* Cancel an in progress work item, if necessary */
> + /* Cancel the in progress task and waiting tasks, 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);
> }
>
> /**
> @@ -423,25 +399,13 @@ struct pci_doe_mb *pcim_doe_create_mb(struct pci_dev *pdev, u16 cap_offset)
> doe_mb->pdev = pdev;
> doe_mb->cap_offset = cap_offset;
> init_waitqueue_head(&doe_mb->wq);
> + mutex_init(&doe_mb->exec_lock);
>
> xa_init(&doe_mb->prots);
> rc = devm_add_action(dev, pci_doe_xa_destroy, doe_mb);
> 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 +460,22 @@ 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
> - *
> - * 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: task to be run
> *
> - * 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
> + * Context: non-interrupt
> + *
> + * 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 +492,18 @@ 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);
> +
> +again:
> + if (!mutex_trylock(&doe_mb->exec_lock)) {
Nit, lock data, not code. This is not a lock of exec_task() it is a lock of
the doe register state. So I would just call it doe_mb->lock.
> + if (wait_event_timeout(task->doe_mb->wq,
At the risk of going all the way back to the original proposal, again
apologies for my premature async advocacy, I do not understand why this
chooses to have a trylock+wait_event instead of just queuing in
mutex_lock_interruptible()?
The mutex will attempt to maintain fairness in its own waitqueue. If the
current task in exec_task() sees PCI_DOE_FLAG_CANCEL, it will drop out
and release the lock and then all waiters can check PCI_DOE_FLAG_CANCEL
before exec_task().
At a minimum this needs a comment about why the built-in mutex waitqueue
is not sufficient for this case. Otherwise, this looks like open-coded
locking to me.
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