Re: [PATCH v03] powerpc/mobility: Fix node detach/rename problem

[Frank Rowand]

Hi Michael Bringmann,

On 12/11/18 8:07 AM, Rob Herring wrote:
> On Tue, Dec 11, 2018 at 7:29 AM Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
>>
>> Hi Michael,
>>
>> Please Cc the device tree folks on device tree patches, and also the
>> original author of the patch that added the code you're modifying.
>>
>> So I've added:
>>   robh+dt@xxxxxxxxxx
>>   frowand.list@xxxxxxxxx
>>   devicetree@xxxxxxxxxxxxxxx
>>   linux-kernel@xxxxxxxxxxxxxxx
>>
>> Michael Bringmann <mwb@xxxxxxxxxxxxxxxxxx> writes:
>>> The PPC mobility code receives RTAS requests to delete nodes with
>>> platform-/hardware-specific attributes when restarting the kernel
>>> after a migration.  My example is for migration between a P8 Alpine
>>> and a P8 Brazos.   Nodes to be deleted include 'ibm,random-v1',
>>> 'ibm,platform-facilities', 'ibm,sym-encryption-v1', and,
>>> 'ibm,compression-v1'.
>>>
>>> The mobility.c code calls 'of_detach_node' for the nodes and their
>>> children.  This makes calls to detach the properties and to remove
>>> the associated sysfs/kernfs files.
>>>
>>> Then new copies of the same nodes are next provided by the PHYP,
>>> local copies are built, and a pointer to the 'struct device_node'
>>> is passed to of_attach_node.  Before the call to of_attach_node,
>>> the phandle is initialized to 0 when the data structure is alloced.
>>> During the call to of_attach_node, it calls __of_attach_node which
>>> pulls the actual name and phandle from just created sub-properties
>>> named something like 'name' and 'ibm,phandle'.
>>>
>>> This is all fine for the first migration.  The problem occurs with
>>> the second and subsequent migrations when the PHYP on the new system
>>> wants to replace the same set of nodes again, referenced with the
>>> same names and phandle values.
>>>
>>> On the second and subsequent migrations, the PHYP tells the system
>>> to again delete the nodes 'ibm,platform-facilities', 'ibm,random-v1',
>>> 'ibm,compression-v1', 'ibm,sym-encryption-v1'.  It specifies these
>>> nodes by its known set of phandle values -- the same handles used
>>> by the PHYP on the source system are known on the target system.
>>> The mobility.c code calls of_find_node_by_phandle() with these values
>>> and ends up locating the first instance of each node that was added
>>> during the original boot, instead of the second instance of each node
>>> created after the first migration.  The detach during the second
>>> migration fails with errors like,
>>>
>>> [ 4565.030704] WARNING: CPU: 3 PID: 4787 at drivers/of/dynamic.c:252 __of_detach_node+0x8/0xa0
>>> [ 4565.030708] Modules linked in: nfsv3 nfs_acl nfs tcp_diag udp_diag inet_diag unix_diag af_packet_diag netlink_diag lockd grace fscache sunrpc xts vmx_crypto sg pseries_rng binfmt_misc ip_tables xfs libcrc32c sd_mod ibmveth ibmvscsi scsi_transport_srp dm_mirror dm_region_hash dm_log dm_mod
>>> [ 4565.030733] CPU: 3 PID: 4787 Comm: drmgr Tainted: G        W         4.18.0-rc1-wi107836-v05-120+ #201
>>> [ 4565.030737] NIP:  c0000000007c1ea8 LR: c0000000007c1fb4 CTR: 0000000000655170
>>> [ 4565.030741] REGS: c0000003f302b690 TRAP: 0700   Tainted: G        W          (4.18.0-rc1-wi107836-v05-120+)
>>> [ 4565.030745] MSR:  800000010282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE,TM[E]>  CR: 22288822  XER: 0000000a
>>> [ 4565.030757] CFAR: c0000000007c1fb0 IRQMASK: 1
>>> [ 4565.030757] GPR00: c0000000007c1fa4 c0000003f302b910 c00000000114bf00 c0000003ffff8e68
>>> [ 4565.030757] GPR04: 0000000000000001 ffffffffffffffff 800000c008e0b4b8 ffffffffffffffff
>>> [ 4565.030757] GPR08: 0000000000000000 0000000000000001 0000000080000003 0000000000002843
>>> [ 4565.030757] GPR12: 0000000000008800 c00000001ec9ae00 0000000040000000 0000000000000000
>>> [ 4565.030757] GPR16: 0000000000000000 0000000000000008 0000000000000000 00000000f6ffffff
>>> [ 4565.030757] GPR20: 0000000000000007 0000000000000000 c0000003e9f1f034 0000000000000001
>>> [ 4565.030757] GPR24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>>> [ 4565.030757] GPR28: c000000001549d28 c000000001134828 c0000003ffff8e68 c0000003f302b930
>>> [ 4565.030804] NIP [c0000000007c1ea8] __of_detach_node+0x8/0xa0
>>> [ 4565.030808] LR [c0000000007c1fb4] of_detach_node+0x74/0xd0
>>> [ 4565.030811] Call Trace:
>>> [ 4565.030815] [c0000003f302b910] [c0000000007c1fa4] of_detach_node+0x64/0xd0 (unreliable)
>>> [ 4565.030821] [c0000003f302b980] [c0000000000c33c4] dlpar_detach_node+0xb4/0x150
>>> [ 4565.030826] [c0000003f302ba10] [c0000000000c3ffc] delete_dt_node+0x3c/0x80
>>> [ 4565.030831] [c0000003f302ba40] [c0000000000c4380] pseries_devicetree_update+0x150/0x4f0
>>> [ 4565.030836] [c0000003f302bb70] [c0000000000c479c] post_mobility_fixup+0x7c/0xf0
>>> [ 4565.030841] [c0000003f302bbe0] [c0000000000c4908] migration_store+0xf8/0x130
>>> [ 4565.030847] [c0000003f302bc70] [c000000000998160] kobj_attr_store+0x30/0x60
>>> [ 4565.030852] [c0000003f302bc90] [c000000000412f14] sysfs_kf_write+0x64/0xa0
>>> [ 4565.030857] [c0000003f302bcb0] [c000000000411cac] kernfs_fop_write+0x16c/0x240
>>> [ 4565.030862] [c0000003f302bd00] [c000000000355f20] __vfs_write+0x40/0x220
>>> [ 4565.030867] [c0000003f302bd90] [c000000000356358] vfs_write+0xc8/0x240
>>> [ 4565.030872] [c0000003f302bde0] [c0000000003566cc] ksys_write+0x5c/0x100
>>> [ 4565.030880] [c0000003f302be30] [c00000000000b288] system_call+0x5c/0x70
>>> [ 4565.030884] Instruction dump:
>>> [ 4565.030887] 38210070 38600000 e8010010 eb61ffd8 eb81ffe0 eba1ffe8 ebc1fff0 ebe1fff8
>>> [ 4565.030895] 7c0803a6 4e800020 e9230098 7929f7e2 <0b090000> 2f890000 4cde0020 e9030040
>>> [ 4565.030903] ---[ end trace 5bd54cb1df9d2976 ]---
>>>
>>> The mobility.c code continues on during the second migration, accepts
>>> the definitions of the new nodes from the PHYP and ends up renaming
>>> the new properties e.g.
>>>
>>> [ 4565.827296] Duplicate name in base, renamed to "ibm,platform-facilities#1"
>>>
>>> There is no check like 'of_node_check_flag(np, OF_DETACHED)' within
>>> of_find_node_by_phandle to skip nodes that are detached, but still
>>> present due to caching or use count considerations.  Also, note that
>>> of_find_node_by_phandle also uses a 'phandle_cache' which does not
>>> appear to be updated when of_detach_node() is invoked.
>>
>> This seems like the real bug. Since the phandle cache was added we can
>> now find detached nodes when we shouldn't be able to.
>>
>> Does the patch below work?
>>
>> cheers
>>
>> diff --git a/drivers/of/base.c b/drivers/of/base.c
>> index 09692c9b32a7..d8e4534c0686 100644
>> --- a/drivers/of/base.c
>> +++ b/drivers/of/base.c
>> @@ -1190,6 +1190,10 @@ 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 we find a detached node, remove it */
>> +               if (of_node_check_flag(np, OF_DETACHED))
>> +                       np = phandle_cache[masked_handle] = NULL;

The bug you found exposes a couple of different issues, a little bit
deeper than the proposed fix.  I'll work on a fuller fix tonight or
tomorrow.


> I'm wondering if we should explicitly remove the node from the cache
> when we set OF_DETACHED. Otherwise, it could be possible that the node
> pointer has been freed already. Or maybe we need both?

Yes, it should be explicitly removed.  I may also add in a paranoia check in
of_find_node_by_phandle().

-Frank

> 
> Rob
>