Re: [PATCH 5/6] cxl: Pass fops and shutdown to memdev creation

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Jul 15, 2021 at 12:41 PM Ben Widawsky <ben.widawsky@intel.com> wrote:
>
> Drivers that use cxl_core for registering a cxl_memdev will likely
> require synchronization with the core for shutdown so as to not race
> against the device going away.

This isn't quite right. There are three independent lifetimes to worry
about: the liveness of the file_operations (bounded by userspace last
close), the driver attach lifetime (bounded by ->remove()), and the
device lifetime (bounded by last put_device()). The devm vs
file_operations lifetime collision is a common problem [1]. I'd go so
far as to define something like:

struct devres_file_operations {
    struct file_operations *fops;
    (int)(*shutdown)(struct device *);
};

...where the definition of @shutdown is:

"invoked in the devres release path to disconnect any driver instance
data from @dev. It assumes synchronization with any fops operation
that requires driver data. After @shutdown an operation may only
reference @device data."

I'd define it locally for now, unless / until other subsystems want to
share the CXL device allocation scheme.

[1]: https://lore.kernel.org/r/YOagA4bgdGYos5aa@kroah.com

> The main example currently is with the
> ioctl interface. Through the duration of the ioctl it's expected that
> the underlying memdev will not disappear.

It's not cxlmd that's the concern, it's cxlm.

> Additionally, it may be desirable to have the fops be passed along as
> well for drivers which do not want the standard handler for the
> character device's ioctl.

Let's give a concrete example. "The unit test code for CXL will also
use a mock driver that provides its own devres_file_operations
instance."

I'll take a shot at reworking this.

>
> As memdev is being migrated to core, this separation must be made.
>
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> ---
>  drivers/cxl/mem.h |  2 ++
>  drivers/cxl/pci.c | 23 +++++++++++++++--------
>  2 files changed, 17 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/cxl/mem.h b/drivers/cxl/mem.h
> index 2092f86beeb8..2b7481376621 100644
> --- a/drivers/cxl/mem.h
> +++ b/drivers/cxl/mem.h
> @@ -42,12 +42,14 @@ extern int cxl_mem_major;
>   * @cdev: char dev core object for ioctl operations
>   * @cxlm: pointer to the parent device driver data
>   * @id: id number of this memdev instance.
> + * @shutdown: Optional function to call on memory device shutdown.
>   */
>  struct cxl_memdev {
>         struct device dev;
>         struct cdev cdev;
>         struct cxl_mem *cxlm;
>         int id;
> +       void (*shutdown)(struct cxl_memdev *cxlmd);
>  };
>
>  /**
> diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> index 6f10b19c9c83..418ae0eac188 100644
> --- a/drivers/cxl/pci.c
> +++ b/drivers/cxl/pci.c
> @@ -1293,11 +1293,13 @@ static void cxl_memdev_unregister(void *_cxlmd)
>         struct device *dev = &cxlmd->dev;
>
>         cdev_device_del(&cxlmd->cdev, dev);
> -       cxl_memdev_shutdown(cxlmd);
> +       if (cxlmd->shutdown)
> +               cxlmd->shutdown(cxlmd);
>         put_device(dev);
>  }
>
> -static struct cxl_memdev *cxl_memdev_alloc(struct cxl_mem *cxlm)
> +static struct cxl_memdev *cxl_memdev_alloc(struct cxl_mem *cxlm,
> +                                          const struct file_operations *fops)
>  {
>         struct pci_dev *pdev = cxlm->pdev;
>         struct cxl_memdev *cxlmd;
> @@ -1323,7 +1325,7 @@ static struct cxl_memdev *cxl_memdev_alloc(struct cxl_mem *cxlm)
>         device_set_pm_not_required(dev);
>
>         cdev = &cxlmd->cdev;
> -       cdev_init(cdev, &cxl_memdev_fops);
> +       cdev_init(cdev, fops);
>         return cxlmd;
>
>  err:
> @@ -1331,15 +1333,17 @@ static struct cxl_memdev *cxl_memdev_alloc(struct cxl_mem *cxlm)
>         return ERR_PTR(rc);
>  }
>
> -static struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
> -                                             struct cxl_mem *cxlm)
> +static struct cxl_memdev *
> +devm_cxl_add_memdev(struct device *host, struct cxl_mem *cxlm,
> +                   const struct file_operations *fops,
> +                   void (*shutdown)(struct cxl_memdev *cxlmd))
>  {
>         struct cxl_memdev *cxlmd;
>         struct device *dev;
>         struct cdev *cdev;
>         int rc;
>
> -       cxlmd = cxl_memdev_alloc(cxlm);
> +       cxlmd = cxl_memdev_alloc(cxlm, fops);
>         if (IS_ERR(cxlmd))
>                 return cxlmd;
>
> @@ -1362,6 +1366,7 @@ static struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
>         rc = devm_add_action_or_reset(host, cxl_memdev_unregister, cxlmd);
>         if (rc)
>                 return ERR_PTR(rc);
> +       cxlmd->shutdown = shutdown;
>         return cxlmd;
>
>  err:
> @@ -1369,7 +1374,8 @@ static struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
>          * The cdev was briefly live, shutdown any ioctl operations that
>          * saw that state.
>          */
> -       cxl_memdev_shutdown(cxlmd);
> +       if (shutdown)
> +               shutdown(cxlmd);
>         put_device(dev);
>         return ERR_PTR(rc);
>  }
> @@ -1610,7 +1616,8 @@ static int cxl_mem_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>         if (rc)
>                 return rc;
>
> -       cxlmd = devm_cxl_add_memdev(&pdev->dev, cxlm);
> +       cxlmd = devm_cxl_add_memdev(&pdev->dev, cxlm, &cxl_memdev_fops,
> +                                   cxl_memdev_shutdown);
>         if (IS_ERR(cxlmd))
>                 return PTR_ERR(cxlmd);
>
> --
> 2.32.0
>