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

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Date:   Wed, 31 Mar 2021 10:07:20 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
        vishal.l.verma@...el.com, ira.weiny@...el.com,
        alison.schofield@...el.com
Subject: Re: [PATCH v3 2/4] cxl/mem: Fix synchronization mechanism for device
 removal vs ioctl operations

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@...dia.com>

Jason