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 10:31:15 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...dia.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 10:03 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> 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);
>
> ?

device_is_registered() inside __cxl_memdev_ioctl().

> 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.

The required barriers are there. srcu_read_lock() +
device_is_registered() is paired with cdev_device_del() +
synchronize_rcu().

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

This usage of srcu is functionally equivalent to replacing
srcu_read_lock() with down_read() and the shutdown path with:

cdev_device_del(...);
down_write(...):
up_write(...);

...to flush readers that read ->state_in_sysfs == 1.