linux-kernel - Re: [PATCH v2 2/4] cxl/mem: Fix synchronization mechanism for device removal vs ioctl operations

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Date:   Tue, 30 Mar 2021 08:16:20 -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 v2 2/4] cxl/mem: Fix synchronization mechanism for device
 removal vs ioctl operations

On Mon, Mar 29, 2021 at 07:47:49PM -0700, Dan Williams wrote:

> @@ -1155,21 +1175,12 @@ static void cxlmdev_unregister(void *_cxlmd)
>  	struct cxl_memdev *cxlmd = _cxlmd;
>  	struct device *dev = &cxlmd->dev;
>  
> -	percpu_ref_kill(&cxlmd->ops_active);
>  	cdev_device_del(&cxlmd->cdev, dev);
> -	wait_for_completion(&cxlmd->ops_dead);
> +	synchronize_srcu(&cxl_memdev_srcu);

This needs some kind of rcu protected pointer for SRCU to to
work.. The write side has to null the pointer and the read side has to
copy the pointer to the stack and check for NULL. 

Otherwise the read side can't detect when the write side is shutting
down.

Basically you must use rcu_derference(), rcu_assign_pointer(), etc
when working with RCU.

Something like 'ops' is usually a reasonable choice

This really can't just be a simple rwsem?

Jason