Re: srcu hung task panic

From: Paul E. McKenney
Date: Tue Oct 23 2018 - 10:14:27 EST

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On Mon, Oct 22, 2018 at 8:11 AM Krein, Dennis <Dennis.Krein@xxxxxxxxxx> wrote:
> Hi Paul,
>
> My name is Dennis Krein - I am a software engineer at Netapp/Solidfire. I
> was referred to you by a tech lead in my departement, Marshalll McMullen.
> We have encountered a bug in Linux kernel - lately occurring in 4.14.67,
> but seen earlier in 4.13.15. This is a hung task panic that comes up in
> synchronize_srcu() where the callback to complete never gets called.
>
> I have tracked this down to a corrupted srcu_data structure for one of the
> CPUs. See attached debug notes for details - dump_201809261610.txt. The
> problem is with the srcu_callback contained in the srcu_data. The head
> points to a callback_head, but all the tails entries point to head. I have
> gone over the code countless times and I can't determine a way that this
> can happen. It almost seems like a memory barrier type of issue.
>
>
> I have managed to come up with a repro scenario. In the attached tgz file
> there is a program called demo_inotify. I use that along with srcutorture
> and can reproduce the hung task panic in a few hours pretty reliably.
>
> Here are the steps
>
>
> - run rcutorture - modprobe rcutorture torture_type=srcu
> - Run the demo_inotify program in a shell loop
> - while 1 do ./demo_ifotify foo
>
>
> I have done this repro scenario 4 times and have hit the bug within 12
> hours or less each time - once in only 2 hours.
>
>
> I am in the process of putting in some bug checks and other
> instrumentation - any help you might be able to provide would be greatly
> appreciated. This appears to be a known bug. I have found instances of
> this on the kernel mailing list, such as
> https://lkml.org/lkml/2018/4/19/669

Have you tried running your tests with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y?
If not, could you please try this? It can be very helpful in finding
issues with the callback lists.

Also, could you please send your .config file?

And yes, the pointers in your rcu_data structure's callback list do
indeed look odd.

Thanx, Paul

u64$ crash2 vmlinux-ember-x86_64-4.14.67-solidfire1 B304-191_sodium-11.0.0.740_09262018_163802_node_bundle.B304-191/crash/dump.201809261610
crash 7.2.0++
...
WARNING: kernel relocated [832MB]: patching 82981 gdb minimal_symbol values

WARNING: kernel version inconsistency between vmlinux and dumpfile

KERNEL: vmlinux-ember-x86_64-4.14.67-solidfire1
DUMPFILE: B304-191_sodium-11.0.0.740_09262018_163802_node_bundle.B304-191/crash/dump.201809261610 [PARTIAL DUMP]
CPUS: 56
DATE: Wed Sep 26 10:09:52 2018
UPTIME: 21:20:03
LOAD AVERAGE: 117.82, 109.58, 96.44
TASKS: 2340
NODENAME: B304-191
RELEASE: 4.14.67-solidfire1
VERSION: #1 SMP Mon Sep 24 17:14:08 UTC 2018
MACHINE: x86_64 (2200 Mhz)
MEMORY: 702.7 GB
PANIC: "Kernel panic - not syncing: hung_task: blocked tasks"
PID: 350
COMMAND: "khungtaskd"
TASK: ffff9d2672f60000 [THREAD_INFO: ffff9d2672f60000]
CPU: 32
STATE: TASK_RUNNING (PANIC)

crash> bt
PID: 350 TASK: ffff9d2672f60000 CPU: 32 COMMAND: "khungtaskd"
#0 [ffffb26259807d30] machine_kexec at ffffffffb503e4bf
#1 [ffffb26259807d88] __crash_kexec at ffffffffb50e5cee
#2 [ffffb26259807e40] panic at ffffffffb5060a83
#3 [ffffb26259807ec0] watchdog at ffffffffb5112c47
#4 [ffffb26259807f10] kthread at ffffffffb507efa9
#5 [ffffb26259807f50] ret_from_fork at ffffffffb5800205

+++++++++++++++++++++++ from dmesg
[76800.726654] INFO: task systemd:1 blocked for more than 60 seconds.
[76800.726657] Tainted: G OE 4.14.67-solidfire1 #1
[76800.726657] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[76800.726659] systemd D 0 1 0 0x00000000
[76800.726662] Call Trace:
[76800.726673] ? __schedule+0x27f/0x870
[76800.726676] schedule+0x28/0x80
[76800.726679] schedule_timeout+0x1e7/0x340
[76800.726685] ? check_preempt_wakeup+0x102/0x230
[76800.726687] ? wait_for_completion+0xb0/0x120
[76800.726689] wait_for_completion+0xb0/0x120
[76800.726693] ? wake_up_q+0x70/0x70
[76800.726698] flush_work+0x10d/0x1c0
[76800.726700] ? worker_detach_from_pool+0xa0/0xa0
[76800.726706] fsnotify_destroy_group+0x34/0xa0
[76800.726708] ? SyS_epoll_ctl+0x1d4/0xe50
[76800.726710] inotify_release+0x1a/0x50
[76800.726714] __fput+0xd8/0x220
[76800.726717] task_work_run+0x8a/0xb0
[76800.726721] exit_to_usermode_loop+0xb9/0xc0
[76800.726723] do_syscall_64+0x10b/0x120
[76800.726727] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[76800.726730] RIP: 0033:0x7fb6957ff900
[76800.726731] RSP: 002b:00007ffc685fdd60 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
[76800.726733] RAX: 0000000000000000 RBX: 0000000000000012 RCX: 00007fb6957ff900
[76800.726735] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000012
[76800.726736] RBP: 00007fb697167088 R08: 000055ae6c9224c0 R09: 000055ae6ace92ad
[76800.726737] R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000000
[76800.726738] R13: 0000000000000000 R14: 0000000000079de4 R15: 0000000000000000
[76800.727130] INFO: task kworker/u113:1:29214 blocked for more than 60 seconds.

crash> set 29214
PID: 29214
COMMAND: "kworker/u113:1"
TASK: ffff9d2673f70000 [THREAD_INFO: ffff9d2673f70000]
CPU: 5
STATE: TASK_UNINTERRUPTIBLE
crash> bt
PID: 29214 TASK: ffff9d2673f70000 CPU: 5 COMMAND: "kworker/u113:1"
#0 [ffffb2627ff07ca0] __schedule at ffffffffb5765eaf
#1 [ffffb2627ff07d30] schedule at ffffffffb57664c8
#2 [ffffb2627ff07d38] schedule_timeout at ffffffffb5769e37
#3 [ffffb2627ff07dc0] wait_for_completion at ffffffffb5766e90
#4 [ffffb2627ff07e08] __synchronize_srcu at ffffffffb50bd6c6
#5 [ffffb2627ff07e70] fsnotify_mark_destroy_workfn at ffffffffb5249e57
#6 [ffffb2627ff07e98] process_one_work at ffffffffb507941a
#7 [ffffb2627ff07ed8] worker_thread at ffffffffb507982f
#8 [ffffb2627ff07f10] kthread at ffffffffb507efa9
#9 [ffffb2627ff07f50] ret_from_fork at ffffffffb5800205

====================================================================== full stack
crash> bt -f
PID: 29214 TASK: ffff9d2673f70000 CPU: 5 COMMAND: "kworker/u113:1"
#0 [ffffb2627ff07ca0] __schedule at ffffffffb5765eaf
ffffb2627ff07ca8: 0000000000000000 ffff9d2673f70000
ffffb2627ff07cb8: ffff9d26732f8000 ffff9d2680b60fc0
ffffb2627ff07cc8: ffffb2627ff07d28 ffffffffb5765eaf
ffffb2627ff07cd8: ffff9d2680ea1028 0000000073f70080
ffffb2627ff07ce8: ffff9d2680b60fc0 ffff9d2680ea0fc0
ffffb2627ff07cf8: ffff9d2600000004 7fffffffffffffff
ffffb2627ff07d08: ffffb2627ff07e20 0000000000000000
ffffb2627ff07d18: 0ffff9d2673f4200 ffff9d2671a395c0
ffffb2627ff07d28: ffffb2627ff07db0 ffffffffb57664c8
#1 [ffffb2627ff07d30] schedule at ffffffffb57664c8
ffffb2627ff07d38: ffffffffb5769e37
#2 [ffffb2627ff07d38] schedule_timeout at ffffffffb5769e37
ffffb2627ff07d40: ffffffffb58001a0 ffffffffb50929c1
ffffb2627ff07d50: ffff9d2680b61028 ffff9d26732f8080
ffffb2627ff07d60: ffff9d2673f70080 0000000000000009
ffffb2627ff07d70: 0000000000000000 0000000000020fc0
ffffb2627ff07d80: ffffb2627ff07e28 ffffb2627ff07dc8
ffffb2627ff07d90: ffffb2627ff07e20 0000000000000000
ffffb2627ff07da0: 0ffff9d2673f4200 ffff9d2671a395c0
ffffb2627ff07db0: 7fffffffffffffff ffffffffb5766e90
ffffb2627ff07dc0: ffffffffb5766e90
#3 [ffffb2627ff07dc0] wait_for_completion at ffffffffb5766e90
ffffb2627ff07dc8: 0000000000000001 ffff9d2673f70000
ffffb2627ff07dd8: ffffffffb5089380 ffffb2627ff07e30
ffffb2627ff07de8: ffffb2627ff07e30 0000000000000001
ffffb2627ff07df8: ffffb2627ff07e60 ffffffffb66bae60 <=== old %rbp == ffffb2627ff07e60 (struct srcu_struct *)sp == ffffffffb66bae60
ffffb2627ff07e08: ffffffffb50bd6c6
#4 [ffffb2627ff07e08] __synchronize_srcu at ffffffffb50bd6c6
ffffb2627ff07e10: 0000000000000000 ffffffffb50bbb50
ffffb2627ff07e20: ffff9d2600000000 ffff9d2600000000
ffffb2627ff07e30: ffffb2627ff07de0 ffffb2627ff07de0
ffffb2627ff07e40: ffffffffb5088ed4 ffffffffb608db60
ffffb2627ff07e50: ffffb2627ff07e78 ffff9d2673f42000
ffffb2627ff07e60: ffffb2627ff07e78 ffffffffb5249e57
ffffb2627ff07e70: ffffffffb5249e57
#5 [ffffb2627ff07e70] fsnotify_mark_destroy_workfn at ffffffffb5249e57
ffffb2627ff07e78: ffff9d0e6ed9c1a0 ffff9d0cfbbf5420
ffffb2627ff07e88: ffffffffb608db60 ffff9d268042fc00 <=== work == ffffffffb608db60
ffffb2627ff07e98: ffffffffb507941a
#6 [ffffb2627ff07e98] process_one_work at ffffffffb507941a
ffffb2627ff07ea0: 000000008042fc00 ffff9d268042fc00
ffffb2627ff07eb0: ffff9d268042fc00 ffff9d268042fc20
ffffb2627ff07ec0: ffff9d2671a395f0 ffff9d268042fc78
ffffb2627ff07ed0: ffff9d2671a395c0 ffffffffb507982f <=== worker == ffff9d2671a395c0
#7 [ffffb2627ff07ed8] worker_thread at ffffffffb507982f
ffffb2627ff07ee0: ffff9d2666dde480 ffff9d82d540d940
ffffb2627ff07ef0: ffffb2627606fdc8 ffff9d2673f70000
ffffb2627ff07f00: ffff9d2671a395c0 ffffffffb5079610
ffffb2627ff07f10: ffffffffb507efa9
#8 [ffffb2627ff07f10] kthread at ffffffffb507efa9
ffffb2627ff07f18: ffff9d2666dde4b8 ffffffffb507ee90
ffffb2627ff07f28: 0000000000000000 ffff9d82d540d940
ffffb2627ff07f38: 0000000000000000 0000000000000000
ffffb2627ff07f48: 0000000000000000 ffffffffb5800205
#9 [ffffb2627ff07f50] ret_from_fork at ffffffffb5800205

crash> p *((struct worker *)0xffff9d2671a395c0)
$3 = {
{
entry = {
next = 0x0,
prev = 0xffff9d268042fcb0
},
hentry = {
next = 0x0,
pprev = 0xffff9d268042fcb0
}
},
current_work = 0xffffffffb608db60, <=== matches work from stack
current_func = 0xffffffffb5249df0,
current_pwq = 0xffff9d2673f42000,
desc_valid = false,
scheduled = {
next = 0xffffb2624006fdf8,
prev = 0xffffffffb608db68
},
task = 0xffff9d2673f70000,
pool = 0xffff9d268042fc00,
node = {
next = 0xffff9d26730fe950,
prev = 0xffff9d268042fed0
},
last_active = 4302606550, <=== 11062 jiffies (110.62 secs) ago
flags = 128,
id = 1,
desc = "flush-259:12\000\000\000\000\000\000\000\000\000\000\000",
rescue_wq = 0x0
}
crash> p jiffies
jiffies = $4 = 4302617612
crash> p/x *((struct work_struct *)0xffffffffb608db60)
$6 = {
data = {
counter = 0xffff9d2673f42005
},
entry = {
next = 0xffff9d2671a395f0,
prev = 0xffffb2624006fdf8
},
func = 0xffffffffb5249df0 <=== fsnotify_mark_destroy_workfn
}
crash> p &((struct work_struct *)0xffffffffb608db60)->entry
$7 = (struct list_head *) 0xffffffffb608db68 <=== came from worker->scheduled
crash> list -H 0xffffffffb608db68 | wc -l
2
crash> p &fsnotify_mark_srcu
$11 = (struct srcu_struct *) 0xffffffffb66bae60 <=== matches sp from stack

====================================================================== rcu
crash> p ((struct rcu_synchronize *)(0xffffb2627ff07e60-0x50)) <=== ffffb2627ff07e60 is saved %rbp from stack
$9 = (struct rcu_synchronize *) 0xffffb2627ff07e10
crash> p *((struct rcu_synchronize *) 0xffffb2627ff07e10)
$10 = {
head = {
next = 0x0,
func = 0xffffffffb50bbb50 <=== wakeme_after_rcu
},
completion = {
done = 0,
wait = {
lock = {
{
rlock = {
raw_lock = {
val = {
counter = 0
}
}
}
}
},
head = {
next = 0xffffb2627ff07de0,
prev = 0xffffb2627ff07de0
}
}
}
}
crash> sym 0xffffffffb50bbb50
ffffffffb50bbb50 (T) wakeme_after_rcu
crash> search 0xffffb2627ff07e10
ffff9d2680b66748: ffffb2627ff07e10 <=== on srcu callback list +++ see below
crash> p fsnotify_mark_srcu.sda
$13 = (struct srcu_data *) 0x26700
crash> p/x __per_cpu_offset[5] <=== hung task is on CPU 5
$14 = 0xffff9d2680b40000
crash> p ((struct srcu_data *)(0xffff9d2680b40000+0x26700))
$15 = (struct srcu_data *) 0xffff9d2680b66700
crash> eval 0xffff9d2680b66748-0xffff9d2680b66700
hexadecimal: 48
crash> srcu_data.srcu_cblist -x
struct srcu_data {
[0x48] struct rcu_segcblist srcu_cblist;
}
crash> p &((struct srcu_data *) 0xffff9d2680b66700)->srcu_cblist.head
$20 = (struct callback_head **) 0xffff9d2680b66748
crash> p *((struct srcu_data *) 0xffff9d2680b66700)
$16 = {
srcu_lock_count = {619057, 595119},
srcu_unlock_count = {619057, 595119},
lock = {
raw_lock = {
val = {
counter = 0
}
}
},
srcu_cblist = {
head = 0xffffb2627ff07e10,
tails = {0xffff9d2680b66748, 0xffff9d2680b66748, 0xffff9d2680b66748, 0xffff9d2680b66748}, <=== all point to head
gp_seq = {0, 56844, 56844, 0},
len = 1, <=== only 1 waiting
len_lazy = 0
},
srcu_gp_seq_needed = 56840,
srcu_gp_seq_needed_exp = 56836,
srcu_cblist_invoking = false,
work = {
work = {
data = {
counter = 3616
},
entry = {
next = 0xffff9d2680b667c0, <=== empty
prev = 0xffff9d2680b667c0
},
func = 0xffffffffb50bcd20 <=== srcu_invoke_callbacks
},
timer = {
entry = {
next = 0xdead000000000200,
pprev = 0x0
},
expires = 4302606551, <=== 11061 jiffies (110.61 secs) ago
function = 0xffffffffb50778c0, <=== delayed_work_timer_fn
data = 18446635387426596792,
flags = 102760453
},
wq = 0xffff9d2680411800,
cpu = 5
},
srcu_barrier_head = {
next = 0x0,
func = 0x0
},
mynode = 0xffffffffb66baec0,
grpmask = 32,
cpu = 5,
sp = 0xffffffffb66bae60
}

======================================================================
crash> dis worker_thread 15
0xffffffffb5079610 <worker_thread>: nopl 0x0(%rax,%rax,1) [FTRACE NOP]
0xffffffffb5079615 <worker_thread+5>: push %r15
0xffffffffb5079617 <worker_thread+7>: mov %rdi,%r15 <=== worker in %r15
0xffffffffb507961a <worker_thread+10>: push %r14
0xffffffffb507961c <worker_thread+12>: push %r13
0xffffffffb507961e <worker_thread+14>: push %r12
0xffffffffb5079620 <worker_thread+16>: push %rbp
0xffffffffb5079621 <worker_thread+17>: push %rbx
0xffffffffb5079622 <worker_thread+18>: mov 0x40(%rdi),%rax
0xffffffffb5079626 <worker_thread+22>: mov 0x48(%rdi),%rbx
0xffffffffb507962a <worker_thread+26>: orl $0x20,0x24(%rax)
0xffffffffb507962e <worker_thread+30>: jmpq 0xffffffffb50796c3 <worker_thread+179>
0xffffffffb5079633 <worker_thread+35>: mov %r15,%rdi
0xffffffffb5079636 <worker_thread+38>: callq 0xffffffffb5079240 <process_one_work>
0xffffffffb507963b <worker_thread+43>: mov 0x30(%r15),%rax

crash> dis process_one_work 15
0xffffffffb5079240 <process_one_work>: nopl 0x0(%rax,%rax,1) [FTRACE NOP]
0xffffffffb5079245 <process_one_work+5>: push %r15 <=== worker on stack
0xffffffffb5079247 <process_one_work+7>: push %r14
0xffffffffb5079249 <process_one_work+9>: push %r13
0xffffffffb507924b <process_one_work+11>: push %r12
0xffffffffb507924d <process_one_work+13>: push %rbp
0xffffffffb507924e <process_one_work+14>: push %rbx
0xffffffffb507924f <process_one_work+15>: mov %rsi,%rbx <=== work in %rbx
0xffffffffb5079252 <process_one_work+18>: sub $0x8,%rsp
0xffffffffb5079256 <process_one_work+22>: mov (%rsi),%rax
0xffffffffb5079259 <process_one_work+25>: mov 0x48(%rdi),%rbp
0xffffffffb507925d <process_one_work+29>: mov %rax,%r12
0xffffffffb5079260 <process_one_work+32>: xor %r12b,%r12b
0xffffffffb5079263 <process_one_work+35>: test $0x4,%al
0xffffffffb5079265 <process_one_work+37>: mov $0x0,%eax

crash> dis fsnotify_mark_destroy_workfn 15
0xffffffffb5249df0 <fsnotify_mark_destroy_workfn>: nopl 0x0(%rax,%rax,1) [FTRACE NOP]
0xffffffffb5249df5 <fsnotify_mark_destroy_workfn+5>: push %rbp
0xffffffffb5249df6 <fsnotify_mark_destroy_workfn+6>: mov $0xffffffffb66bb308,%rdi
0xffffffffb5249dfd <fsnotify_mark_destroy_workfn+13>: push %rbx <=== work on stack
0xffffffffb5249dfe <fsnotify_mark_destroy_workfn+14>: sub $0x10,%rsp
0xffffffffb5249e02 <fsnotify_mark_destroy_workfn+18>: callq 0xffffffffb576b530 <_raw_spin_lock>
0xffffffffb5249e07 <fsnotify_mark_destroy_workfn+23>: mov 0xe43db2(%rip),%rdx # 0xffffffffb608dbc0
0xffffffffb5249e0e <fsnotify_mark_destroy_workfn+30>: mov $0xffffffffb608dbc0,%rax
0xffffffffb5249e15 <fsnotify_mark_destroy_workfn+37>: mov $0xffffffffb66bb308,%rdi
0xffffffffb5249e1c <fsnotify_mark_destroy_workfn+44>: mov %rsp,%rbp
0xffffffffb5249e1f <fsnotify_mark_destroy_workfn+47>: mov %rbp,0x8(%rdx)
0xffffffffb5249e23 <fsnotify_mark_destroy_workfn+51>: mov %rdx,(%rsp)
0xffffffffb5249e27 <fsnotify_mark_destroy_workfn+55>: mov 0xe43d9a(%rip),%rdx # 0xffffffffb608dbc8
0xffffffffb5249e2e <fsnotify_mark_destroy_workfn+62>: mov %rdx,0x8(%rsp)
0xffffffffb5249e33 <fsnotify_mark_destroy_workfn+67>: mov %rbp,(%rdx)

crash> dis -l __synchronize_srcu
/usr/src/linux/kernel/rcu/srcutree.c: 881
0xffffffffb50bd650 <__synchronize_srcu>: nopl 0x0(%rax,%rax,1) [FTRACE NOP]
0xffffffffb50bd655 <__synchronize_srcu+5>: lea 0x8(%rsp),%r10
0xffffffffb50bd65a <__synchronize_srcu+10>: and $0xfffffffffffffff0,%rsp
0xffffffffb50bd65e <__synchronize_srcu+14>: pushq -0x8(%r10)
0xffffffffb50bd662 <__synchronize_srcu+18>: push %rbp
0xffffffffb50bd663 <__synchronize_srcu+19>: mov %rsp,%rbp
0xffffffffb50bd666 <__synchronize_srcu+22>: push %r12
0xffffffffb50bd668 <__synchronize_srcu+24>: mov %rdi,%r12 <=== sp in %r12
0xffffffffb50bd66b <__synchronize_srcu+27>: push %r10
0xffffffffb50bd66d <__synchronize_srcu+29>: push %rbx
0xffffffffb50bd66e <__synchronize_srcu+30>: mov %esi,%ebx
0xffffffffb50bd670 <__synchronize_srcu+32>: sub $0x38,%rsp
/usr/src/linux/kernel/rcu/srcutree.c: 893
0xffffffffb50bd674 <__synchronize_srcu+36>: callq 0xffffffffb57666c0 <_cond_resched>
/usr/src/linux/kernel/rcu/srcutree.c: 894
0xffffffffb50bd679 <__synchronize_srcu+41>: mov %r12,%rdi
0xffffffffb50bd67c <__synchronize_srcu+44>: callq 0xffffffffb50bc9d0 <check_init_srcu_struct>
/usr/src/linux/./include/linux/completion.h: 120
0xffffffffb50bd681 <__synchronize_srcu+49>: lea -0x50(%rbp),%rax
0xffffffffb50bd685 <__synchronize_srcu+53>: mov $0xffffffffb6658124,%rdx
0xffffffffb50bd68c <__synchronize_srcu+60>: mov $0xffffffffb5e23732,%rsi
0xffffffffb50bd693 <__synchronize_srcu+67>: lea 0x18(%rax),%rdi
/usr/src/linux/./include/linux/completion.h: 119
0xffffffffb50bd697 <__synchronize_srcu+71>: movl $0x0,-0x40(%rbp)
/usr/src/linux/./include/linux/completion.h: 120
0xffffffffb50bd69e <__synchronize_srcu+78>: callq 0xffffffffb50a01b0 <__init_waitqueue_head>
/usr/src/linux/kernel/rcu/srcutree.c: 897
0xffffffffb50bd6a3 <__synchronize_srcu+83>: lea -0x50(%rbp),%rsi <=== &rcu-head
0xffffffffb50bd6a7 <__synchronize_srcu+87>: movzbl %bl,%ecx
0xffffffffb50bd6aa <__synchronize_srcu+90>: mov %r12,%rdi
0xffffffffb50bd6ad <__synchronize_srcu+93>: mov $0xffffffffb50bbb50,%rdx
0xffffffffb50bd6b4 <__synchronize_srcu+100>: callq 0xffffffffb50bd2f0 <__call_srcu>
/usr/src/linux/kernel/rcu/srcutree.c: 898
0xffffffffb50bd6b9 <__synchronize_srcu+105>: lea -0x50(%rbp),%rax
0xffffffffb50bd6bd <__synchronize_srcu+109>: lea 0x10(%rax),%rdi
0xffffffffb50bd6c1 <__synchronize_srcu+113>: callq 0xffffffffb5766de0 <wait_for_completion>
/usr/src/linux/kernel/rcu/srcutree.c: 908
0xffffffffb50bd6c6 <__synchronize_srcu+118>: mfence <=== RA
/usr/src/linux/kernel/rcu/srcutree.c: 909
0xffffffffb50bd6c9 <__synchronize_srcu+121>: add $0x38,%rsp
0xffffffffb50bd6cd <__synchronize_srcu+125>: pop %rbx
0xffffffffb50bd6ce <__synchronize_srcu+126>: pop %r10
0xffffffffb50bd6d0 <__synchronize_srcu+128>: pop %r12
0xffffffffb50bd6d2 <__synchronize_srcu+130>: pop %rbp
0xffffffffb50bd6d3 <__synchronize_srcu+131>: lea -0x8(%r10),%rsp
0xffffffffb50bd6d7 <__synchronize_srcu+135>: retq
0xffffffffb50bd6d8 <__synchronize_srcu+136>: nopl 0x0(%rax,%rax,1)

crash> dis wait_for_completion 15
0xffffffffb5766de0 <wait_for_completion>: nopl 0x0(%rax,%rax,1) [FTRACE NOP]
0xffffffffb5766de5 <wait_for_completion+5>: push %r12 <=== sp on stack
0xffffffffb5766de7 <wait_for_completion+7>: mov %rdi,%r12
0xffffffffb5766dea <wait_for_completion+10>: push %rbp <=== %rbp on stack
0xffffffffb5766deb <wait_for_completion+11>: push %rbx
0xffffffffb5766dec <wait_for_completion+12>: lea 0x8(%r12),%rbx
0xffffffffb5766df1 <wait_for_completion+17>: sub $0x28,%rsp
0xffffffffb5766df5 <wait_for_completion+21>: callq 0xffffffffb57666c0 <_cond_resched>
0xffffffffb5766dfa <wait_for_completion+26>: mov %rbx,%rdi
0xffffffffb5766dfd <wait_for_completion+29>: callq 0xffffffffb576b550 <_raw_spin_lock_irq>
0xffffffffb5766e02 <wait_for_completion+34>: mov (%r12),%eax
0xffffffffb5766e06 <wait_for_completion+38>: test %eax,%eax
0xffffffffb5766e08 <wait_for_completion+40>: jne 0xffffffffb5766ed6 <wait_for_completion+246>
0xffffffffb5766e0e <wait_for_completion+46>: mov %gs:0x14d00,%rax
0xffffffffb5766e17 <wait_for_completion+55>: mov %rax,0x8(%rsp)

----- End forwarded message -----

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