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 14:02:53 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-cxl@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "Schofield, Alison" <alison.schofield@...el.com>
Subject: Re: [PATCH v2 2/4] cxl/mem: Fix synchronization mechanism for device
 removal vs ioctl operations

On Tue, Mar 30, 2021 at 09:05:29AM -0700, Dan Williams wrote:

> > If you can't clearly point to the *data* under RCU protection it is
> > being used wrong.
> 
> Agree.
> 
> The data being protected is the value of
> dev->kobj.state_in_sysfs. The

So where is that read under:

+	idx = srcu_read_lock(&cxl_memdev_srcu);
+	rc = __cxl_memdev_ioctl(cxlmd, cmd, arg);
+	srcu_read_unlock(&cxl_memdev_srcu, idx);

?

It can't read the RCU protected data outside the RCU critical region,
and it can't read/write RCU protected data without using the helper
macros which insert the required barriers.

IMHO this can't use 'dev->kobj.state_in_sysfs' as the RCU protected data.

Jason