[PATCH 15/16] mtd: mtdcore: fix initcall level

[PATCH 15/16] mtd: mtdcore: fix initcall level

[Alexander Holler]

The mtd-core has to be initialized before other dependent mtd-drivers,
otherwise a crash might occur.

Currently mtd_init() is called in the initcall-level device, which is the
same level where most mtd-drivers will end up. By luck this seemed to have
been called most of the time before other mtd-drivers without having been
explicitly enforced. But if mtd_init() is not called before a dependent
driver, a null-pointer exception might occur (e.g. because the mtd device
class isn't registered).

To fix this, mtd-init() is moved to the initcall-level fs (right before
the standard initcall level device).

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
---
 drivers/mtd/mtdcore.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 8bbbb75..fa8e6452 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -1303,7 +1303,7 @@ static void __exit cleanup_mtd(void)
 	bdi_destroy(&mtd_bdi);
 }
 
-module_init(init_mtd);
+fs_initcall_sync(init_mtd);
 module_exit(cleanup_mtd);
 
 MODULE_LICENSE("GPL");
-- 
2.1.0

Re: [PATCH 15/16] mtd: mtdcore: fix initcall level

[Brian Norris]

Hi Alexander,

No judgment here for the rest of this series, but for this patch:

On Wed, Aug 26, 2015 at 02:28:27PM +0200, Alexander Holler wrote:
> The mtd-core has to be initialized before other dependent mtd-drivers,
> otherwise a crash might occur.
> 
> Currently mtd_init() is called in the initcall-level device, which is the
> same level where most mtd-drivers will end up. By luck this seemed to have
> been called most of the time before other mtd-drivers without having been
> explicitly enforced.

I can't really speak for the original authors, but it does not appear to
be entirely "by luck." Link order was one of the de facto ways to get
this ordering (though it's not really a great one), and mtdcore was
always linked first within the drivers/mtd/ directory structure.

But that's just background, I think this is worth fixing anyway. It
could, for instance, become a problem if drivers are located outside
drivers/mtd/; I see random board files in arch/ that register with MTD,
and I'm actually not sure how they have never tripped on this.

> But if mtd_init() is not called before a dependent
> driver, a null-pointer exception might occur (e.g. because the mtd device
> class isn't registered).
> 
> To fix this, mtd-init() is moved to the initcall-level fs (right before
> the standard initcall level device).

Is there a good reason we shouldn't just make this a subsys_initcall()?
That would match the naming better, and it mirrors what, e.g.,
block/genhd uses. It would also allow flexibility if there are other
current/future use cases that might need to sit between the subsystem
and the drivers.

> Signed-off-by: Alexander Holler <holler@ahsoftware.de>
> ---
>  drivers/mtd/mtdcore.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
> index 8bbbb75..fa8e6452 100644
> --- a/drivers/mtd/mtdcore.c
> +++ b/drivers/mtd/mtdcore.c
> @@ -1303,7 +1303,7 @@ static void __exit cleanup_mtd(void)
>  	bdi_destroy(&mtd_bdi);
>  }
>  
> -module_init(init_mtd);
> +fs_initcall_sync(init_mtd);

Why the *_sync() version? init_mtd() is very simple and doesn't have any
multithreading issues to handle.

>  module_exit(cleanup_mtd);
>  
>  MODULE_LICENSE("GPL");
> -- 
> 2.1.0
> 

Regards,
Brian

Re: [PATCH 15/16] mtd: mtdcore: fix initcall level

[Alexander Holler]

Am 01.09.2015 um 23:19 schrieb Brian Norris:
> Hi Alexander,
>
> No judgment here for the rest of this series, but for this patch:
>
> On Wed, Aug 26, 2015 at 02:28:27PM +0200, Alexander Holler wrote:
>> The mtd-core has to be initialized before other dependent mtd-drivers,
>> otherwise a crash might occur.
>>
>> Currently mtd_init() is called in the initcall-level device, which is the
>> same level where most mtd-drivers will end up. By luck this seemed to have
>> been called most of the time before other mtd-drivers without having been
>> explicitly enforced.
>
> I can't really speak for the original authors, but it does not appear to
> be entirely "by luck." Link order was one of the de facto ways to get
> this ordering (though it's not really a great one), and mtdcore was
> always linked first within the drivers/mtd/ directory structure.
>
> But that's just background, I think this is worth fixing anyway. It
> could, for instance, become a problem if drivers are located outside
> drivers/mtd/; I see random board files in arch/ that register with MTD,
> and I'm actually not sure how they have never tripped on this.

I've already found at least a half a dozen other drivers with the same 
problem through my shuffling of the drivers which all end up in the 
standard initcall level device. I'm aware that this is based on the link 
order, which itself is based on linker behaviour (and maybe other things 
like make or other build tools). I'm just calling it luck, because this 
is nowhere explicitly stated, at least I've never seen such a statement, 
neither in any source, nor somewhere in Documentation. So I've choosen 
the term "by luck" in order to provoke a bit (or to stimulate a 
discussion about that widespread problem).

>
>> But if mtd_init() is not called before a dependent
>> driver, a null-pointer exception might occur (e.g. because the mtd device
>> class isn't registered).
>>
>> To fix this, mtd-init() is moved to the initcall-level fs (right before
>> the standard initcall level device).
>
> Is there a good reason we shouldn't just make this a subsys_initcall()?
> That would match the naming better, and it mirrors what, e.g.,
> block/genhd uses. It would also allow flexibility if there are other
> current/future use cases that might need to sit between the subsystem
> and the drivers.

No real reason here. The names for the initcall levels seem to be 
outdated since a long time anyway, and I think just speaking about the 
numbers 1-7 (or 0-14) would be better anyways. The only reason why I've 
used the fs (sync) level is because I was too lazy to check out if 
mtdcore might depend on something else in any other level. Therefor I've 
used the one most close to were it was before.

Also I've an idea about how to fix that in general for all drivers (by 
using the same algorithm I've now introduced just for DT-based drivers 
with a device description). Wouldn't be that hard to use that for all 
drivers, but as I've written in a follow up to the introductory mail, 
one step after another.

Regards,

Alexander Holler

Re: [PATCH 15/16] mtd: mtdcore: fix initcall level

[Alexander Holler]

Am 02.09.2015 um 07:34 schrieb Alexander Holler:
> Am 01.09.2015 um 23:19 schrieb Brian Norris:
>> Hi Alexander,
>>
>> No judgment here for the rest of this series, but for this patch:
>>
>> On Wed, Aug 26, 2015 at 02:28:27PM +0200, Alexander Holler wrote:
>>> The mtd-core has to be initialized before other dependent mtd-drivers,
>>> otherwise a crash might occur.
>>>
>>> Currently mtd_init() is called in the initcall-level device, which is
>>> the
>>> same level where most mtd-drivers will end up. By luck this seemed to
>>> have
>>> been called most of the time before other mtd-drivers without having
>>> been
>>> explicitly enforced.
>>
>> I can't really speak for the original authors, but it does not appear to
>> be entirely "by luck." Link order was one of the de facto ways to get
>> this ordering (though it's not really a great one), and mtdcore was
>> always linked first within the drivers/mtd/ directory structure.
>>
>> But that's just background, I think this is worth fixing anyway. It
>> could, for instance, become a problem if drivers are located outside
>> drivers/mtd/; I see random board files in arch/ that register with MTD,
>> and I'm actually not sure how they have never tripped on this.
>
> I've already found at least a half a dozen other drivers with the same
> problem through my shuffling of the drivers which all end up in the
> standard initcall level device. I'm aware that this is based on the link
> order, which itself is based on linker behaviour (and maybe other things
> like make or other build tools). I'm just calling it luck, because this
> is nowhere explicitly stated, at least I've never seen such a statement,
> neither in any source, nor somewhere in Documentation. So I've choosen
> the term "by luck" in order to provoke a bit (or to stimulate a
> discussion about that widespread problem).

A good example why "luck" might not be far away from the truth is what 
happens when a drivers moves e.g. from staging to it's real position. 
Also the position will stay inside the same initcall level, the move of 
the driver into another directory (maybe together with a rename) will 
likely change its position in the initcall-sequence, without anyone 
really expecting this.

>>> But if mtd_init() is not called before a dependent
>>> driver, a null-pointer exception might occur (e.g. because the mtd
>>> device
>>> class isn't registered).
>>>
>>> To fix this, mtd-init() is moved to the initcall-level fs (right before
>>> the standard initcall level device).
>>
>> Is there a good reason we shouldn't just make this a subsys_initcall()?
>> That would match the naming better, and it mirrors what, e.g.,
>> block/genhd uses. It would also allow flexibility if there are other
>> current/future use cases that might need to sit between the subsystem
>> and the drivers.
>
> No real reason here. The names for the initcall levels seem to be
> outdated since a long time anyway, and I think just speaking about the
> numbers 1-7 (or 0-14) would be better anyways. The only reason why I've
> used the fs (sync) level is because I was too lazy to check out if
> mtdcore might depend on something else in any other level. Therefor I've
> used the one most close to were it was before.

Lazy was the wrong term. It is time consuming, cumbersome and therefor 
error-prone to check on what other stuff a driver depends. One reason 
why choosing the right place in the initcall sequence isn't that easy 
and why some automation make sense.

> Also I've an idea about how to fix that in general for all drivers (by
> using the same algorithm I've now introduced just for DT-based drivers
> with a device description). Wouldn't be that hard to use that for all
> drivers, but as I've written in a follow up to the introductory mail,
> one step after another.
>
> Regards,
>
> Alexander Holler

Re: [PATCH 15/16] mtd: mtdcore: fix initcall level

[Alexander Holler]

Am 04.09.2015 um 06:00 schrieb Alexander Holler:
> Am 02.09.2015 um 07:34 schrieb Alexander Holler:
>> Am 01.09.2015 um 23:19 schrieb Brian Norris:
>>> Hi Alexander,
>>>
>>> No judgment here for the rest of this series, but for this patch:
>>>
>>> On Wed, Aug 26, 2015 at 02:28:27PM +0200, Alexander Holler wrote:
>>>> The mtd-core has to be initialized before other dependent mtd-drivers,
>>>> otherwise a crash might occur.
>>>>
>>>> Currently mtd_init() is called in the initcall-level device, which is
>>>> the
>>>> same level where most mtd-drivers will end up. By luck this seemed to
>>>> have
>>>> been called most of the time before other mtd-drivers without having
>>>> been
>>>> explicitly enforced.
>>>
>>> I can't really speak for the original authors, but it does not appear to
>>> be entirely "by luck." Link order was one of the de facto ways to get
>>> this ordering (though it's not really a great one), and mtdcore was
>>> always linked first within the drivers/mtd/ directory structure.
>>>
>>> But that's just background, I think this is worth fixing anyway. It
>>> could, for instance, become a problem if drivers are located outside
>>> drivers/mtd/; I see random board files in arch/ that register with MTD,
>>> and I'm actually not sure how they have never tripped on this.

As I've just had a look at my patches in order to clean up the patch for 
parallel initialization (to post it here too):

drivers/mtd/ofparts.c has the same problem. In order to let the 
NAND-driver see the partitions defined in the DT I had to move this into 
another initcall level (fs sync) too.

Regards,

Alexander Holler