linux-kernel - Re: [PATCH 2/2] of: __of_detach_node()

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Date:   Fri, 14 Dec 2018 15:56:42 -0600
From:   Michael Bringmann <mwb@...ux.vnet.ibm.com>
To:     Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>,
        tlfalcon@...ux.vnet.ibm.com, minkim@...ibm.com,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] of: __of_detach_node() - remove node from phandle
 cache

On 12/14/2018 11:20 AM, Rob Herring wrote:
> On Fri, Dec 14, 2018 at 12:43 AM <frowand.list@...il.com> wrote:
>>
>> From: Frank Rowand <frank.rowand@...y.com>
>>
>> Non-overlay dynamic devicetree node removal may leave the node in
>> the phandle cache.  Subsequent calls to of_find_node_by_phandle()
>> will incorrectly find the stale entry.  Remove the node from the
>> cache.
>>
>> Add paranoia checks in of_find_node_by_phandle() as a second level
>> of defense (do not return cached node if detached, do not add node
>> to cache if detached).
>>
>> Reported-by: Michael Bringmann <mwb@...ux.vnet.ibm.com>
>> Signed-off-by: Frank Rowand <frank.rowand@...y.com>
>> ---
>>  drivers/of/base.c       | 29 ++++++++++++++++++++++++++++-
>>  drivers/of/dynamic.c    |  3 +++
>>  drivers/of/of_private.h |  4 ++++
>>  3 files changed, 35 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/of/base.c b/drivers/of/base.c
>> index d599367cb92a..34a5125713c8 100644
>> --- a/drivers/of/base.c
>> +++ b/drivers/of/base.c
>> @@ -162,6 +162,27 @@ int of_free_phandle_cache(void)
>>  late_initcall_sync(of_free_phandle_cache);
>>  #endif
>>
>> +/*
>> + * Caller must hold devtree_lock.
>> + */
>> +void __of_free_phandle_cache_entry(phandle handle)
>> +{
>> +       phandle masked_handle;
>> +
>> +       if (!handle)
>> +               return;
>> +
>> +       masked_handle = handle & phandle_cache_mask;
>> +
>> +       if (phandle_cache) {
>> +               if (phandle_cache[masked_handle] &&
>> +                   handle == phandle_cache[masked_handle]->phandle) {
>> +                       of_node_put(phandle_cache[masked_handle]);
>> +                       phandle_cache[masked_handle] = NULL;
>> +               }
>> +       }
>> +}
>> +
>>  void of_populate_phandle_cache(void)
>>  {
>>         unsigned long flags;
>> @@ -1209,11 +1230,17 @@ struct device_node *of_find_node_by_phandle(phandle handle)
>>                 if (phandle_cache[masked_handle] &&
>>                     handle == phandle_cache[masked_handle]->phandle)
>>                         np = phandle_cache[masked_handle];
>> +               if (np && of_node_check_flag(np, OF_DETACHED)) {
>> +                       of_node_put(np);
>> +                       phandle_cache[masked_handle] = NULL;
> 
> This should never happen, right? Any time we set OF_DETACHED, the
> entry should get removed from the cache. I think we want a WARN here
> in case we're in an unexpected state.

We don't actually remove the pointer from the phandle cache when we set
OF_DETACHED in drivers/of/dynamic.c:__of_detach_node.  The phandle cache
is currently static within drivers/of/base.c.  There are a couple of
calls to of_populate_phandle_cache / of_free_phandle_cache within
drivers/of/overlay.c, but these are not involved in the device tree
updates that occur during LPAR migration.  A WARN here would only make
sense, if we also arrange to clear the handle.

> 
> Rob

Michael

> 
> 

-- 
Michael W. Bringmann
Linux I/O, Networking and Security Development
IBM Corporation
Tie-Line  363-5196
External: (512) 286-5196
Cell:       (512) 466-0650
mwb@...ux.vnet.ibm.com