RE: [PATCH 1/2] mtd: use refcount to prevent corruption

["Usyskin, Alexander" <alexander.usyskin@xxxxxxxxx>]

Hi
> 
> Hi Miquel,
> >
> > Hi Alexander,
> >
> > alexander.usyskin@xxxxxxxxx wrote on Mon, 24 Jul 2023 11:43:59 +0000:
> >
> > > > > > > With this patch applied, when I load up the module, I get the same 3
> > > > > > > devices:
> > > > > > > /dev/mtd0
> > > > > > > /dev/mtd0ro
> > > > > > > /dev/mtdblock0
> > > > > > >
> > > > > > > Upon removal, the below 2 devices still hang around:
> > > > > > > /dev/mtd0
> > > > > > > /dev/mtd0ro
> > > > > >
> > > > > Our use-case do not produce mtdblock, maybe there are some
> > imbalances
> > > > of get/put?
> > > > > I have somewhere version with pr_debug after every kref_get/put. That
> > may
> > > > help to catch where
> > > > > it missed, I hope.
> > > >
> > > > I believe mtdblock is the good citizen here. Just disable
> > > > CONFIG_MTD_BLOCK from your configuration and you will likely observe
> > > > the same issue, just a bit narrowed, perhaps. Indeed, if you manage to
> > > > follow all the get/put calls it can help to find an imbalance.
> > > >
> > > > Thanks,
> > > > Miquèl
> > >
> > > Miquel, do you have CONFIG_MTD_PARTITIONED_MASTER set in your
> > config?
> >
> > Not sure I get your question. You can enable or disable it, it should
> > work in both cases (yet, the handling is of course a bit different as
> > the top level device will be retained/not retained).
> >
> > Thanks,
> > Miquèl
> 
> I'm trying to understand why I can't reproduce the problem in my scenario.
> I found an important difference in upstreamed patch and internal version:
> The IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER) check around
> kref_get/put does not exists in the internal tree.
> The code before my patch do not have such check, so I tend to assume that
> this check should be removed.
> If you reproduce happens with CONFIG_MTD_PARTITIONED_MASTER
> disabled that may explain problems that you see.
> 
> --
> Thanks,
> Sasha
> 

I've tried to reproduce this with latest Linux 6.5-rc1 and my two patches.
The manual modprobe mtdblock creates mtdblock0 over my partitions too.
I can't reproduce problem neither with MTD_PARTITIONED_MASTER nor without.

Let's try to debug on your system, can you enable dynamic debug for mtd subsystem,
reproduce and publish dmesg?

The prints for kref get/put can be added as below:

diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 2466ea466466..374835831428 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -1242,10 +1242,13 @@ int __get_mtd_device(struct mtd_info *mtd)
        }

        kref_get(&mtd->refcnt);
+       pr_debug("get mtd %s %d\n", mtd->name, kref_read(&mtd->refcnt));

        while (mtd->parent) {
-               if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER) || mtd->parent != master)
+               if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER) || mtd->parent != master) {
                        kref_get(&mtd->parent->refcnt);
+                       pr_debug("get mtd %s %d\n", mtd->parent->name, kref_read(&mtd->parent->refcnt));
+               }
                mtd = mtd->parent;
        }

@@ -1335,12 +1338,15 @@ void __put_mtd_device(struct mtd_info *mtd)
        while (mtd != master) {
                struct mtd_info *parent = mtd->parent;

+               pr_debug("put mtd %s %d\n", mtd->name, kref_read(&mtd->refcnt));
                kref_put(&mtd->refcnt, mtd_device_release);
                mtd = parent;
        }

-       if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER))
+       if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER)) {
+               pr_debug("put mtd %s %d\n", master->name, kref_read(&master->refcnt));
                kref_put(&master->refcnt, mtd_device_release);
+       }

        module_put(master->owner);


-- 
Thanks,
Sasha