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

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Message-ID: <20210330175431.GX2356281@nvidia.com>
Date:   Tue, 30 Mar 2021 14:54:31 -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 10:31:15AM -0700, Dan Williams wrote:
> 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().

Oh, I see, I missed that

> > 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().

RCU needs barriers on the actual load/store just a naked
device_is_registered() alone is not strong enough.

> > 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:

Sort of, but the rules for load/store under RCU are different than for
load/store under a normal barriered lock. All the data is unstable for
instance and minimially needs READ_ONCE.

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

The lock would have to enclose the store to state_in_sysfs, otherwise
as written it has the same data race problems.

Jason