linux-kernel - Re: [PATCH v2 17/22] drivers/base: Make device_find_child_by

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Date:   Mon, 13 Jul 2020 08:39:43 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Joao Martins <joao.m.martins@...cle.com>
Subject: Re: [PATCH v2 17/22] drivers/base: Make device_find_child_by_name()
 compatible with sysfs inputs

On Sun, Jul 12, 2020 at 10:09 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Sun, Jul 12, 2020 at 09:27:37AM -0700, Dan Williams wrote:
> > Use sysfs_streq() in device_find_child_by_name() to allow it to use a
> > sysfs input string that might contain a trailing newline.
> >
> > The other "device by name" interfaces,
> > {bus,driver,class}_find_device_by_name(), already account for sysfs
> > strings.
> >
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> > ---
> >  drivers/base/core.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index 67d39a90b45c..5d31b962c898 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -3078,7 +3078,7 @@ struct device *device_find_child_by_name(struct device *parent,
> >
> >       klist_iter_init(&parent->p->klist_children, &i);
> >       while ((child = next_device(&i)))
> > -             if (!strcmp(dev_name(child), name) && get_device(child))
> > +             if (sysfs_streq(dev_name(child), name) && get_device(child))
>
> Who wants to call this function with a name passed from userspace?
>
> Not objecting to it, just curious...
>

The series that incorporates this patch adds a partitioning mechanism
to "device-dax region" devices with an:
    "echo 1 > regionX/create" to create a new partition / sub-instance
of a region, and...
    "echo $devname > regionX/delete" to delete. Where $devname is
searched in the child devices of regionX to trigger device_del().

This arrangement avoids one of the design mistakes of libnvdimm which
uses a sysfs attribute of the device to delete itself. Parent-device
triggered deletion rather than self-deletion avoids those locking
entanglements.