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

Wed Aug 1 00:17:50 AEST 2018

On Tue, Jul 31, 2018 at 12:34 AM Michael Ellerman <mpe at ellerman.id.au> wrote:
>
> Hi Rob/Frank,
>
> I think we might have a problem with the phandle_cache not interacting
> well with of_detach_node():

Probably needs a similar fix as this commit did for overlays:

commit b9952b5218added5577e4a3443969bc20884cea9
Author: Frank Rowand <frank.rowand at sony.com>
Date:   Thu Jul 12 14:00:07 2018 -0700

    of: overlay: update phandle cache on overlay apply and remove

    A comment in the review of the patch adding the phandle cache said that
    the cache would have to be updated when modules are applied and removed.
    This patch implements the cache updates.

    Fixes: 0b3ce78e90fc ("of: cache phandle nodes to reduce cost of
of_find_node_by_phandle()")
    Reported-by: Alan Tull <atull at kernel.org>
    Suggested-by: Alan Tull <atull at kernel.org>
    Signed-off-by: Frank Rowand <frank.rowand at sony.com>
    Signed-off-by: Rob Herring <robh at kernel.org>

Really what we need here is an "invalidate phandle" function rather
than free and re-allocate the whole damn cache.

Rob

>
> 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