phandle_cache vs of_detach_node (was Re: [PATCH] powerpc/mobility: Fix node detach/rename problem)

Tue Jul 31 16:34:41 AEST 2018

Hi Rob/Frank,

I think we might have a problem with the phandle_cache not interacting
well with of_detach_node():

Michael Bringmann <mwb at linux.vnet.ibm.com> writes:
> See below.
>
> On 07/30/2018 01:31 AM, Michael Ellerman wrote:
>> Michael Bringmann <mwb at linux.vnet.ibm.com> writes:
>> 
>>> During LPAR migration, the content of the device tree/sysfs may
>>> be updated including deletion and replacement of nodes in the
>>> tree.  When nodes are added to the internal node structures, they
>>> are appended in FIFO order to a list of nodes maintained by the
>>> OF code APIs.
>> 
>> That hasn't been true for several years. The data structure is an n-ary
>> tree. What kernel version are you working on?
>
> Sorry for an error in my description.  I oversimplified based on the
> name of a search iterator.  Let me try to provide a better explanation
> of the problem, here.
>
> This is the problem.  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 may include 'ibm,random-v1',
> 'ibm,compression-v1', 'ibm,platform-facilities', 'ibm,sym-encryption-v1',
> or others.
>
> The mobility.c code calls 'of_detach_node' for the nodes and their children.
> This makes calls to detach the properties and to try 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.
>
>> 
>>> When nodes are removed from the device tree, they
>>> are marked OF_DETACHED, but not actually deleted from the system
>>> to allow for pointers cached elsewhere in the kernel.  The order
>>> and content of the entries in the list of nodes is not altered,
>>> though.
>> 
>> Something is going wrong if this is actually happening.
>> 
>> When the node is detached it should be *detached* from the tree of all
>> nodes, so it should not be discoverable other than by having an existing
>> pointer to it.
> 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"
>
> I don't see any 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.  Another possibility to consider
> is that of_find_node_by_phandle also uses something called 'phandle_cache'
> which may have outdated data as of_detach_node() does not have access to
> that cache for the 'OF_DETACHED' nodes.

Yes the phandle_cache looks like it might be the problem.

I saw of_free_phandle_cache() being called as late_initcall, but didn't
realise that's only if MODULES is disabled.

So I don't see anything that invalidates the phandle_cache when a node
is removed.

The right solution would be for __of_detach_node() to invalidate the
phandle_cache for the node being detached. That's slightly complicated
by the phandle_cache being static inside base.c

To test the theory that it's the phandle_cache causing the problems can
you try this patch:

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 848f549164cd..60e219132e24 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -1098,6 +1098,9 @@ 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 (of_node_check_flag(np, OF_DETACHED))
+			np = NULL;
 	}
 
 	if (!np) {

cheers
