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Subject: Re: of_node_put() usage is buggy all over drivers/of/base.c?!
From:   Frank Rowand <frowand.list@gmail.com>
To:     Rob Herring <robh+dt@kernel.org>,
        Vladimir Oltean <olteanv@gmail.com>
Cc:     Sascha Hauer <s.hauer@pengutronix.de>, devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>
References: <20210814010139.kzryimmp4rizlznt@skbuf>
 <9accd63a-961c-4dab-e167-9e2654917a94@gmail.com>
 <20210816144622.tgslast6sbblclda@skbuf>
 <4cad28e0-d6b4-800d-787b-936ffaca7be3@gmail.com>
 <CAL_JsqKYd288Th2cfOp0_HD1C8xtgKjyJfUW4JLpyn0NkGdU5w@mail.gmail.com>
 <2e98373f-c37c-0d26-5c9a-1f15ade243c1@gmail.com>
Message-ID: <84ccd797-c307-d724-0292-ee74aba43de2@gmail.com>
Date:   Mon, 16 Aug 2021 15:00:51 -0500
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Hit send too soon, a couple of cleanups:

On 8/16/21 2:56 PM, Frank Rowand wrote:
> On 8/16/21 2:20 PM, Rob Herring wrote:
>> On Mon, Aug 16, 2021 at 10:14 AM Frank Rowand <frowand.list@gmail.com> wrote:
>>>
>>> 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.
>>
> 
>> Because it is an iterator function and they all drop the ref from the
>> prior iteration.
> 
> That is what I was expecting before reading through the code.  But instead
> I found of_find_compatible_node():
> 
>         raw_spin_lock_irqsave(&devtree_lock, flags);
>         for_each_of_allnodes_from(from, np)
>                 if (__of_device_is_compatible(np, compatible, type, NULL) &&
>                     of_node_get(np))
>                         break;
>         of_node_put(from);
>         raw_spin_unlock_irqrestore(&devtree_lock, flags);
> 
> 
> for_each_of_allnodes_fromir:

  for_each_of_allnodes_from():

> 
> #define for_each_of_allnodes_from(from, dn) \
>         for (dn = __of_find_all_nodes(from); dn; dn = __of_find_all_nodes(dn))
> 
> 
> and __of_find_all_nodes() is:
> 
> struct device_node *__of_find_all_nodes(struct device_node *prev)
> {
>         struct device_node *np;
>         if (!prev) {
>                 np = of_root;
>         } else if (prev->child) {
>                 np = prev->child;
>         } else {
>                 /* Walk back up looking for a sibling, or the end of the structure */
>                 np = prev;
>                 while (np->parent && !np->sibling)
>                         np = np->parent;
>                 np = np->sibling; /* Might be null at the end of the tree */
>         }
>         return np;
> }
> 
> 
> So the iterator is not using of_node_get() and of_node_put() for each
> node that is traversed.  The protection against a node disappearing
> during the iteration is provided by holding devtree_lock.
> 
>>
>> I would say any open coded call where from is not NULL is an error.
> 
> I assume you mean any open coded call of of_find_compatible_node().  There are
> at least a couple of instances of that.  I did only a partial grep while looking
> at Vladimir's issue.
> 
> Doing the full grep now, I see 13 instances of architecture and driver code calling
> of_find_compatible_node().

  of_find_compatible_node() with parameter "from" not NULL.

> 
>> It's not reliable because the DT search order is not defined and could
>> change. Someone want to write a coccinelle script to check that?
>>
> 
>> The above code should be using of_get_compatible_child() instead.
> 
> Yes, of_get_compatible_child() should be used here.  Thanks for pointing
> that out.
> 
> There are 13 instances of architecture and driver code calling
> of_find_compatible_node().  If possible, it would be good to change all of
> them to of_get_compatible_child().  If we could replace all driver
> usage of of_find_compatible_node() with a from parameter of NULL to
> a new wrapper without a from parameter, where the wrapper calls
> of_find_compatible_node() with the from parameter set to NULL, then
> we could prevent this problem from recurring.
> 
> (I did not look at all 13 instances yet, to see if this can be done.)
> 
>>
>> Rob
>>
>