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

[Miquel Raynal]

Hi Alexander,

alexander.usyskin at intel.com wrote on Tue, 25 Jul 2023 12:50:04 +0000:

> Hi
> > 
> > Hi Miquel,  
> > >
> > > Hi Alexander,
> > >
> > > alexander.usyskin at intel.com 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@huawei.com/

If you successfully test it, please send your Tested-by.

Thanks,
Miquèl