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

[Miquel Raynal]

Hi Tomas,

tomas.winkler@xxxxxxxxx wrote on Thu, 27 Jul 2023 06:32:39 +0000:

> > 
> > Hi Alexander,
> > 
> > alexander.usyskin@xxxxxxxxx wrote on Tue, 25 Jul 2023 12:50:04 +0000:
> > 
> > > 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);
> > >
> > >
> > 
> > Could this be helpful?
> > 
> > https://lore.kernel.org/all/20230725215539.3135304-1-
> > zhangxiaoxu5@xxxxxxxxxx/
> > 
> > If you successfully test it, please send your Tested-by.
> In the first glance it doesn't look correct, we have the reproduced using kasan, so hopefully the fix will follow, shortly. 
> Thanks
> Tomas
> 

Why is this fix not correct?

Are you currently writing a fix yourself?

Thanks,
Miquèl