Re: of_node_put() usage is buggy all over drivers/of/base.c?!

Frank Rowand
2021-08-16T15:14:26Z

On 8/16/21 9:46 AM, Vladimir Oltean wrote:
> Hi Frank,
>
> On Mon, Aug 16, 2021 at 09:33:03AM -0500, Frank Rowand wrote:
>> Hi Vladimir,
>>
>> On 8/13/21 8:01 PM, Vladimir Oltean wrote:
>>> Hi,
>>>
>>> I was debugging an RCU stall which happened during the probing of a
>>> driver. Activating lock debugging, I see:
>>
>> I took a quick look at sja1105_mdiobus_register() in v5.14-rc1 and v5.14-rc6.
>>
>> Looking at the following stack trace, I did not see any calls to
>> of_find_compatible_node() in sja1105_mdiobus_register(). I am
>> guessing that maybe there is an inlined function that calls
>> of_find_compatible_node(). This would likely be either
>> sja1105_mdiobus_base_tx_register() or sja1105_mdioux_base_t1_register().
>
> Yes, it is sja1105_mdiobus_base_t1_register which is inlined.
>
>>>
>>> [ 101.710694] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:938
>>> [ 101.719119] in_atomic(): 1, irqs_disabled(): 128, non_block: 0, pid: 1534, name: sh
>>> [ 101.726763] INFO: lockdep is turned off.
>>> [ 101.730674] irq event stamp: 0
>>> [ 101.733716] hardirqs last enabled at (0): [<0000000000000000>] 0x0
>>> [ 101.739973] hardirqs last disabled at (0): [<ffffd3ebecb10120>] copy_process+0xa78/0x1a98
>>> [ 101.748146] softirqs last enabled at (0): [<ffffd3ebecb10120>] copy_process+0xa78/0x1a98
>>> [ 101.756313] softirqs last disabled at (0): [<0000000000000000>] 0x0
>>> [ 101.762569] CPU: 4 PID: 1534 Comm: sh Not tainted 5.14.0-rc5+ #272
>>> [ 101.774558] Call trace:
>>> [ 101.794734] __might_sleep+0x50/0x88
>>> [ 101.798297] __mutex_lock+0x60/0x938
>>> [ 101.801863] mutex_lock_nested+0x38/0x50
>>> [ 101.805775] kernfs_remove+0x2c/0x50 <---- this takes mutex_lock(&kernfs_mutex);
>>> [ 101.809341] sysfs_remove_dir+0x54/0x70
>>
>> The __kobject_del() occurs only if the refcount on the node
>> becomes zero. This should never be true when of_find_compatible_node()
>> calls of_node_put() unless a "from" node is passed to of_find_compatible_node().
>
> I figured that was the assumption, that the of_node_put would never
> trigger a sysfs file / kobject deletion from there.
>
>> In both sja1105_mdiobus_base_tx_register() and sja1105_mdioux_base_t1_register()
>> a from node ("mdio") is passed to of_find_compatible_node() without first doing an
>> of_node_get(mdio). If you add the of_node_get() calls the problem should be fixed.
>
> The answer seems simple enough, but stupid question, but why does
> of_find_compatible_node call of_node_put on "from" in the first place?

Actually a good question.

I do not know why of_find_compatible_node() calls of_node_put() instead of making
the caller of of_find_compatible_node() responsible. That pattern was created
long before I was involved in devicetree and I have not gone back to read the
review comments of when that code was created.

-Frank