Re: [PATCH v3 2/4] cxl/mem: Fix synchronization mechanism for device removal vs ioctl operations

[Jason Gunthorpe]

On Tue, Mar 30, 2021 at 04:36:37PM -0700, Dan Williams wrote:
> -static void cxlmdev_unregister(void *_cxlmd)
> +static void cxl_memdev_activate(struct cxl_memdev *cxlmd, struct cxl_mem *cxlm)
>  {
> -	struct cxl_memdev *cxlmd = _cxlmd;
> -	struct device *dev = &cxlmd->dev;
> +	cxlmd->cxlm = cxlm;
> +	down_write(&cxl_memdev_rwsem);
> +	up_write(&cxl_memdev_rwsem);
> +}

No reason not to put the assignment inside the lock. Though using the
lock at all is overkill as the pointer hasn't left the local stack
frame at this point.

>  err_add:
> -	ida_free(&cxl_memdev_ida, cxlmd->id);
> -err_id:
>  	/*
> -	 * Theoretically userspace could have already entered the fops,
> -	 * so flush ops_active.
> +	 * The cdev was briefly live, shutdown any ioctl operations that
> +	 * saw that state.
>  	 */

Wow it is really subtle that cdev_device_add has this tiny hole where
it can fail but have already allowed open() :<

Other than the lock it looks OK

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Jason