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SubjectRe: Null pointer crash at find_idlest_group on db845c w/ linus/master
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Hi John,

On 03/12/2019 19:15, John Stultz wrote:
> With today's linus/master on db845c running android, I'm able to
> fairly easily reproduce the following crash. I've not had a chance to
> bisect it yet, but I'm suspecting its connected to Vincent's recent
> rework.
>

Thanks for reporting this.

> If you have any suggestions, or need me to test anything, please let me know.
>
> thanks
> -john
>
> [ 136.157069] Unable to handle kernel NULL pointer dereference at
> virtual address 0000000000000010
> [ 136.165937] Mem abort info:
> [ 136.168765] ESR = 0x96000005
> [ 136.171862] EC = 0x25: DABT (current EL), IL = 32 bits
> [ 136.177229] SET = 0, FnV = 0
> [ 136.180320] EA = 0, S1PTW = 0
> [ 136.183502] Data abort info:
> [ 136.186426] ISV = 0, ISS = 0x00000005
> [ 136.190302] CM = 0, WnR = 0
> [ 136.193316] user pgtable: 4k pages, 39-bit VAs, pgdp=0000000175f7e000
> [ 136.199814] [0000000000000010] pgd=0000000000000000, pud=0000000000000000
> [ 136.206666] Internal error: Oops: 96000005 [#1] PREEMPT SMP
> [ 136.212295] Modules linked in:
> [ 136.215397] CPU: 7 PID: 50 Comm: kauditd Tainted: G W
> 5.4.0-mainline-11225-g9c5add21ff63 #1252
> [ 136.225390] Hardware name: Thundercomm Dragonboard 845c (DT)
> [ 136.231111] pstate: 60c00085 (nZCv daIf +PAN +UAO)
> [ 136.235971] pc : find_idlest_group.isra.95+0x368/0x690
> [ 136.241159] lr : find_idlest_group.isra.95+0x210/0x690
> [ 136.246347] sp : ffffffc01036b890
> [ 136.249708] x29: ffffffc01036b890 x28: ffffffe7d7450480
> [ 136.255077] x27: ffffff80f81f0000 x26: 0000000000000000
> [ 136.260445] x25: 0000000000000000 x24: ffffffc01036b998
> [ 136.265810] x23: ffffff80f8e40a00 x22: ffffffe7d7719e30
> [ 136.271175] x21: ffffff80f8f16520 x20: ffffff80f8f16180
> [ 136.276539] x19: ffffffe7d771a610 x18: ffffffe7d71d1ef0
> [ 136.281908] x17: 0000000000000000 x16: 0000000000000000
> [ 136.287274] x15: 0000000000000001 x14: 6f3a753d74786574
> [ 136.292644] x13: 6e6f637420383637 x12: 632c323135633a30
> [ 136.298007] x11: 0000000000000070 x10: 0000000000000002
> [ 136.303371] x9 : 0000000000000000 x8 : 0000000000000075
> [ 136.308737] x7 : ffffff80f8f16000 x6 : ffffffe7d7450000
> [ 136.314099] x5 : 0000000000000004 x4 : 0000000000000000
> [ 136.319465] x3 : 000000000000025c x2 : ffffff80f8f16780
> [ 136.324836] x1 : 0000000000000000 x0 : 0000000000000002
> [ 136.330198] Call trace:
> [ 136.332680] find_idlest_group.isra.95+0x368/0x690
> [ 136.337528] select_task_rq_fair+0x4e4/0xd88
> [ 136.341848] try_to_wake_up+0x21c/0x7f8
> [ 136.345735] default_wake_function+0x34/0x48
> [ 136.350053] pollwake+0x98/0xc8
> [ 136.353233] __wake_up_common+0x90/0x158
> [ 136.357202] __wake_up_common_lock+0x88/0xd0
> [ 136.361519] __wake_up_sync_key+0x40/0x50
> [ 136.365584] sock_def_readable+0x44/0x78
> [ 136.369560] __netlink_sendskb+0x44/0x58
> [ 136.373525] netlink_unicast+0x220/0x258
> [ 136.377496] kauditd_send_queue+0xa4/0x158
> [ 136.381643] kauditd_thread+0xf4/0x248
> [ 136.385438] kthread+0x130/0x138
> [ 136.388706] ret_from_fork+0x10/0x1c
> [ 136.392332] Code: 54001340 7100081f 540016e1 a9478ba1 (f9400821)
> [ 136.398493] ---[ end trace 47d254973801b2c4 ]---
> [ 136.403162] Kernel panic - not syncing: Fatal exception
> [ 136.408445] SMP: stopping secondary CPUs
> [ 136.412440] Kernel Offset: 0x27c6200000 from 0xffffffc010000000
> [ 136.418417] PHYS_OFFSET: 0xffffffe140000000
> [ 136.422655] CPU features: 0x00002,2a80a218
>

I fed this to decode_stacktrace.sh using your branch, I get:

[ 136.157069] Unable to handle kernel NULL pointer dereference at
virtual address 0000000000000010
[ 136.165937] Mem abort info:
[ 136.168765] ESR = 0x96000005
[ 136.171862] EC = 0x25: DABT (current EL), IL = 32 bits
[ 136.177229] SET = 0, FnV = 0
[ 136.180320] EA = 0, S1PTW = 0
[ 136.183502] Data abort info:
[ 136.186426] ISV = 0, ISS = 0x00000005
[ 136.190302] CM = 0, WnR = 0
[ 136.193316] user pgtable: 4k pages, 39-bit VAs, pgdp=0000000175f7e000
[ 136.199814] [0000000000000010] pgd=0000000000000000, pud=0000000000000000
[ 136.206666] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[ 136.212295] Modules linked in:
[ 136.215397] CPU: 7 PID: 50 Comm: kauditd Tainted: G W
5.4.0-mainline-11225-g9c5add21ff63 #1252
[ 136.225390] Hardware name: Thundercomm Dragonboard 845c (DT)
[ 136.231111] pstate: 60c00085 (nZCv daIf +PAN +UAO)
[ 136.235971] pc : find_idlest_group.isra.95+0x368/0x690
[ 136.241159] lr : find_idlest_group.isra.95+0x210/0x690
[ 136.246347] sp : ffffffc01036b890
[ 136.249708] x29: ffffffc01036b890 x28: ffffffe7d7450480
[ 136.255077] x27: ffffff80f81f0000 x26: 0000000000000000
[ 136.260445] x25: 0000000000000000 x24: ffffffc01036b998
[ 136.265810] x23: ffffff80f8e40a00 x22: ffffffe7d7719e30
[ 136.271175] x21: ffffff80f8f16520 x20: ffffff80f8f16180
[ 136.276539] x19: ffffffe7d771a610 x18: ffffffe7d71d1ef0
[ 136.281908] x17: 0000000000000000 x16: 0000000000000000
[ 136.287274] x15: 0000000000000001 x14: 6f3a753d74786574
[ 136.292644] x13: 6e6f637420383637 x12: 632c323135633a30
[ 136.298007] x11: 0000000000000070 x10: 0000000000000002
[ 136.303371] x9 : 0000000000000000 x8 : 0000000000000075
[ 136.308737] x7 : ffffff80f8f16000 x6 : ffffffe7d7450000
[ 136.314099] x5 : 0000000000000004 x4 : 0000000000000000
[ 136.319465] x3 : 000000000000025c x2 : ffffff80f8f16780
[ 136.324836] x1 : 0000000000000000 x0 : 0000000000000002
[ 136.330198] Call trace:
[ 136.332680] find_idlest_group.isra.95+0x368/0x690
[ 136.337528] select_task_rq_fair (kernel/sched/fair.c:5663 kernel/sched/fair.c:6387)
[ 136.341848] try_to_wake_up (kernel/sched/core.c:2397 kernel/sched/core.c:2644)
[ 136.345735] default_wake_function (kernel/sched/core.c:4350)
[ 136.350053] pollwake (fs/select.c:218)
[ 136.353233] __wake_up_common (kernel/sched/wait.c:93)
[ 136.357202] __wake_up_common_lock (./include/linux/spinlock.h:393 (discriminator 1) kernel/sched/wait.c:125 (discriminator 1))
[ 136.361519] __wake_up_sync_key (kernel/sched/wait.c:191)
[ 136.365584] sock_def_readable (./include/asm-generic/bitops/non-atomic.h:106 ./include/net/sock.h:837 ./include/net/sock.h:2208 net/core/sock.c:2798)
[ 136.369560] __netlink_sendskb (net/netlink/af_netlink.c:1251)
[ 136.373525] netlink_unicast (./include/linux/refcount.h:255 ./include/linux/refcount.h:281 ./include/net/sock.h:1728 net/netlink/af_netlink.c:1257 net/netlink/af_netlink.c:1343)
[ 136.377496] kauditd_send_queue (kernel/audit.c:734)
[ 136.381643] kauditd_thread (kernel/audit.c:858 (discriminator 4))
[ 136.385438] kthread (kernel/kthread.c:684)
[ 136.388706] ret_from_fork (arch/arm64/kernel/entry.S:909)
[ 136.392332] Code: 54001340 7100081f 540016e1 a9478ba1 (f9400821)
All code
========
0: 54001340 b.eq 0x268
4: 7100081f cmp w0, #0x2
8: 540016e1 b.ne 0x2e4
c: a9478ba1 ldp x1, x2, [x29,#120]
10:* f9400821 ldr x1, [x1,#16] <-- trapping instruction

Code starting with the faulting instruction
===========================================
0: f9400821 ldr x1, [x1,#16]
[ 136.398493] ---[ end trace 47d254973801b2c4 ]---
[ 136.403162] Kernel panic - not syncing: Fatal exception
[ 136.408445] SMP: stopping secondary CPUs
[ 136.412440] Kernel Offset: 0x27c6200000 from 0xffffffc010000000
[ 136.418417] PHYS_OFFSET: 0xffffffe140000000

The ISRA makes this a bit annoying to figure out the faulty line.

On a sidenote, I never really looked at what fipa-sra does, so I scrounged
this from GCC:

"""
IPA-SRA is an interprocedural pass that removes unused function return
values (turning functions returning a value which is never used into void
functions), removes unused function parameters. It can also replace an
aggregate parameter by a set of other parameters representing part of the
original, turning those passed by reference into new ones which pass the
value directly.
"""

I suspect this is GCC getting rid of the 'sd_flag' parameter of
find_idlest_group() which is now unused.


Back to the thing, the interesting bit is

0: 54001340 b.eq 0x268
4: 7100081f cmp w0, #0x2
8: 540016e1 b.ne 0x2e4
c: a9478ba1 ldp x1, x2, [x29,#120]
10:* f9400821 ldr x1, [x1,#16] <-- trapping instruction

The misfit task cases seem to match the pattern of the last two lines. Here's
one:

case group_misfit_task:
/* Select group with the highest max capacity */
if (local->sgc->max_capacity >= idlest->sgc->max_capacity)
1754: a94783a2 ldp x2, x0, [x29,#120]
1758: f9400801 ldr x1, [x0,#16]


Looking at the code, I think I got it. In find_idlest_group() we do
initialize 'idlest_sgs' (just like busiest_stat in LB) but 'idlest' is just
NULL. The latter is dereferenced in update_pick_idlest() just for the misfit
case, which goes boom. And I reviewed the damn thing... Bleh.

Fixup looks easy enough, lemme write one up.

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