RE: [PATCH] mtd: ubi: Fix race condition between ubi device creation and udev

["Harutyunov, Iosif" <iharutyunov@SonicWALL.com>]

> -----Original Message-----
> On Thu, 2016-07-21 at 02:07 +0000, Iosif Harutyunov wrote:
> > While implementing udev rules for UBI device I ran into the problem
> > when udev would ignore UBI rules I created to process attachment of
> > volume.
> > Interestingly, when I trigger
> > udev ubi subsystem "change" event manually after the attachment, rules
> > would work just fine.
> >
> > I traced problem down to UBI sysfs processing, which turned out to be
> > racing condition.
> > See patch below to address the problem.
> >
> > Thanks.
> > Iosif,_
> >
> > Signed-off-by: Iosif Harutyunov <iharutyunov@sonicwall.com>
> > ---
> > Once ubi is attached, make sure ubi_devices[] is initialized early
> > before being used in the dev_attribute_show().
> >
> > This is to prevent race condition between udev ubi rules processing
> > and ubi device creation, which manifests itself ignoring udev ATTR
> > rules.
> 
> Good catch!
> 
> Quick feedback: this patch will probably work fine, we could apply the
> following logic and select a better place for that line of code. Here it is:
> 
> 1. Initialize the device, make it usable. Do not make it available to anyone
> before that. This is the code before the 'uif_init()'
> invocation. BTW, "uif" means "user interfaces" in this case.
> 
> 2. Make the device "available" (or "visible") by adding it to the 'ubi_devices'
> array.
> 
> 3. Initialize user interfaces, done by 'uif_init()'.
> 
> IOW, it put that line just before 'uif_init()'.

Sounds good. Thanks for feedback. I have revised patch based on your and
Richard's input. Please find it below.

Regards.
Iosif,_

Signed-off-by: Iosif Harutyunov <iharutyunov@sonicwall.com>
---
Fix race condition between ubi device and udev: make sure ubi_devices[]
is initialized before device becomes accessible to users via sysfs.
---
drivers/mtd/ubi/build.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
index ef36182..5d524a0 100644
--- a/drivers/mtd/ubi/build.c
+++ b/drivers/mtd/ubi/build.c
@@ -992,6 +992,9 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num,
                        goto out_detach;
        }
 
+       /* Make device "available" before it becomes accessible via sysfs */
+       ubi_devices[ubi_num] = ubi;
+
        err = uif_init(ubi, &ref);
        if (err)
                goto out_detach;
@@ -1036,7 +1039,6 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num,
        wake_up_process(ubi->bgt_thread);
        spin_unlock(&ubi->wl_lock);
 
-       ubi_devices[ubi_num] = ubi;
        ubi_notify_all(ubi, UBI_VOLUME_ADDED, NULL);
        return ubi_num;
 
@@ -1047,6 +1049,7 @@ out_uif:
        ubi_assert(ref);
        uif_close(ubi);
 out_detach:
+       ubi_devices[ubi_num] = NULL;
        ubi_wl_close(ubi);
        ubi_free_internal_volumes(ubi);
        vfree(ubi->vtbl);
--
1.7.7.6