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 08:45:23 -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 v3 2/4] cxl/mem: Fix synchronization mechanism for device
 removal vs ioctl operations

On Wed, Mar 31, 2021 at 6:07 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> 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.

Right, I was considering just leaving it as a bare pointer assignment,
in fact that must be sufficient as publishing the cdev needs to depend
on all cdev init having completed. So if this write somehow leaks into
cdev_device_add() there are much larger problems afoot.

>
> >  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() :<

Yes, this was something I wanted to address in the cdev api proposal
integrating the debugfs fops proxy / reference counting aproach. I
want a cdev api that does not allow open until after the associated
device has registered and fired the KOBJ_ADD event.

>
> Other than the lock it looks OK
>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Thanks, Jason.