On Mon, Aug 16, 2021 at 2:57 PM Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>
> On 8/16/21 2:20 PM, Rob Herring wrote:
> > On Mon, Aug 16, 2021 at 10:14 AM Frank Rowand <frowand.list@xxxxxxxxx> 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():
No, I meant of_find_compatible_node() is the iterator for
for_each_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:
>
> #define for_each_of_allnodes_from(from, dn) \
> for (dn = __of_find_all_nodes(from); dn; dn = __of_find_all_nodes(dn))
This is only used internally, so it can rely on the caller holding the
lock. This should be moved into of_private.h.
> 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.
The lock is for traversing the nodes (i.e. a list lock), not keeping
nodes around.
>
> >
> > 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().
>
> > 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.
Patches welcome.
I don't know if all 13 are only looking for child nodes. Could be open
coding for_each_compatible_node or looking for grandchild nodes in
addition (for which we don't have helpers).
Rob
