Hi all,
we're able to trigger the oops below when doing CPU hotplug tests.
Disassembling the code section of the oops gives
0: 1a 00 sbb (%rax),%al
2: b8 64 00 00 00 mov $0x64,%eax
7: 2b 05 5c a4 28 01 sub 0x128a45c(%rip),%eax # 0x128a469
d: be 64 00 00 00 mov $0x64,%esi
12: 31 d2 xor %edx,%edx
14: 8b 7d e0 mov -0x20(%rbp),%edi
17: 48 0f af c3 imul %rbx,%rax
1b: 48 f7 f6 div %rsi
1e: 31 d2 xor %edx,%edx
20: 48 89 c1 mov %rax,%rcx
23: 48 0f af 4d e8 imul -0x18(%rbp),%rcx
28: 48 89 c8 mov %rcx,%rax
2b:* 48 f7 f7 div %rdi <-- trapping instruction
2e: 31 d2 xor %edx,%edx
30: 48 89 c1 mov %rax,%rcx
33: 41 8b 84 24 4c 01 00 mov 0x14c(%r12),%eax
3a: 00
3b: 48 0f af c3 imul %rbx,%rax
3f: 48 rex.W
in bdi_dirty_limit. The .s file contains then (annotations mine):
.globl bdi_dirty_limit
.type bdi_dirty_limit, @function
bdi_dirty_limit:
pushq %rbp #
movq %rsp, %rbp #,
pushq %r12 #
pushq %rbx #
subq $48, %rsp #,
call mcount
movq %rsi, %rbx # dirty, dirty
leaq -32(%rbp), %rcx #, tmp65
leaq -24(%rbp), %rdx #, tmp66
leaq 280(%rdi), %rsi #, tmp67
movq %rdi, %r12 # bdi, bdi
movq $writeout_completions, %rdi #,
call fprop_fraction_percpu #
movl $100, %eax #, tmp69
subl bdi_min_ratio(%rip), %eax # bdi_min_ratio, tmp70
movl $100, %esi #, tmp75
xorl %edx, %edx #
mov -32(%rbp), %edi # denominator, denominator
imulq %rbx, %rax # dirty, tmp71
divq %rsi # tmp75
xorl %edx, %edx # # most-significant part of bdi_dirty is already zeroed here
movq %rax, %rcx # tmp71, tmp73
imulq -24(%rbp), %rcx # numerator, tmp73 # bdi_dirty *= numerator
movq %rcx, %rax # tmp73, # move bdi_dirty in place for next insn
divq %rdi # denominator <--- TRAP
xorl %edx, %edx #
movq %rax, %rcx #, tmp78
...
and from looking at the register dump below, the dividend, which should
be in %rdx:%rax is 0 and the divisor (denominator) we've got from
bdi_writeout_fraction and is in %rdi is also 0. Which is strange because
fprop_fraction_percpu guards for division by zero by setting denominator
to 1 if it were zero but what about the case where den > num? Can that
even happen?
And also, what happens if num is 0? Which it kinda is by looking at %rcx
where there's copy of it.
I'll let people more knowledgeable with the code explain what actually
happens.
unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
{
u64 bdi_dirty;
long numerator, denominator;
/*
* Calculate this BDI's share of the dirty ratio.
*/
bdi_writeout_fraction(bdi, &numerator, &denominator);
bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
bdi_dirty *= numerator;
do_div(bdi_dirty, denominator);
bdi_dirty += (dirty * bdi->min_ratio) / 100;
if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
bdi_dirty = dirty * bdi->max_ratio / 100;
return bdi_dirty;
}
Sep 23 17:41:08 lemure kernel: [ 381.245776] divide error: 0000 [#1] SMP
Sep 23 17:41:08 lemure kernel: [ 381.249725] Modules linked in: cpufreq_conservative cpufreq_userspace cpufreq_powersave i2c_piix4 tpm_tis rtc_cmos powernow_k8 tpm fam15
h_power k10temp tpm_bios mperf serio_raw crc32c_intel ghash_clmulni_intel
Sep 23 17:41:08 lemure kernel: [ 381.268531] CPU 0
Sep 23 17:41:08 lemure kernel: [ 381.270377] Pid: 6644, comm: flush-8:0 Not tainted 3.6.0-rc6-e5e77cf9-linus+ #1 AMD
Sep 23 17:41:08 lemure kernel: [ 381.279067] RIP: 0010:[<ffffffff810e8bc2>] [<ffffffff810e8bc2>] bdi_dirty_limit+0x5a/0x9e
Sep 23 17:41:08 lemure kernel: [ 381.287330] RSP: 0018:ffff88041ad03d40 EFLAGS: 00010246
Sep 23 17:41:08 lemure kernel: [ 381.292631] RAX: 0000000000000000 RBX: 00000000000621c3 RCX: 0000000000000000
Sep 23 17:41:08 lemure kernel: [ 381.299751] RDX: 0000000000000000 RSI: 0000000000000064 RDI: 0000000000000000
Sep 23 17:41:08 lemure kernel: [ 381.306870] RBP: ffff88041ad03d80 R08: 0000000000000008 R09: ffffffff8211e520
Sep 23 17:41:08 lemure kernel: [ 381.313989] R10: ffff88041ad03d10 R11: ffff88041ad03d10 R12: ffff88041a2d0158
Sep 23 17:41:08 lemure kernel: [ 381.321109] R13: ffff88041a2d0158 R14: ffff88041a2d02b0 R15: 0000000000000000
Sep 23 17:41:08 lemure kernel: [ 381.328228] FS: 00007f3db8ea7700(0000) GS:ffff88042ec00000(0000) knlGS:0000000000000000
Sep 23 17:41:08 lemure kernel: [ 381.336298] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
Sep 23 17:41:08 lemure kernel: [ 381.342034] CR2: 0000000000d84270 CR3: 0000000418ce4000 CR4: 00000000000407f0
Sep 23 17:41:08 lemure kernel: [ 381.349151] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Sep 23 17:41:08 lemure kernel: [ 381.356263] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Sep 23 17:41:08 lemure kernel: [ 381.363384] Process flush-8:0 (pid: 6644, threadinfo ffff88041ad02000, task ffff8804198826c0)
Sep 23 17:41:08 lemure kernel: [ 381.371884] Stack:
Sep 23 17:41:08 lemure kernel: [ 381.373890] ffff88041ad03d80 ffffffff810e8e7a 0000000100013eb3 0000000000000000
Sep 23 17:41:08 lemure kernel: [ 381.381330] 2000000000000000 0000000000000000 fffffffffffffff7 0000000000000000
Sep 23 17:41:08 lemure kernel: [ 381.388769] ffff88041ad03dc0 ffffffff8114f9bd 000000010000c983 00000000000c4386
Sep 23 17:41:08 lemure kernel: [ 381.396208] Call Trace:
Sep 23 17:41:09 lemure kernel: [ 381.398654] [<ffffffff810e8e7a>] ? global_dirty_limits+0x3c/0x130
Sep 23 17:41:09 lemure kernel: [ 381.404823] [<ffffffff8114f9bd>] over_bground_thresh+0x5c/0x76
Sep 23 17:41:09 lemure kernel: [ 381.410729] [<ffffffff811503aa>] wb_do_writeback+0x193/0x1e9
Sep 23 17:41:09 lemure kernel: [ 381.416464] [<ffffffff811504ca>] bdi_writeback_thread+0xca/0x1ec
Sep 23 17:41:09 lemure kernel: [ 381.422545] [<ffffffff81150400>] ? wb_do_writeback+0x1e9/0x1e9
Sep 23 17:41:09 lemure kernel: [ 381.428455] [<ffffffff8105e75b>] kthread+0x8d/0x95
Sep 23 17:41:09 lemure kernel: [ 381.433323] [<ffffffff81940474>] kernel_thread_helper+0x4/0x10
Sep 23 17:41:09 lemure kernel: [ 381.439231] [<ffffffff8105e6ce>] ? kthread_freezable_should_stop+0x62/0x62
Sep 23 17:41:09 lemure kernel: [ 381.446178] [<ffffffff81940470>] ? gs_change+0xb/0xb
Sep 23 17:41:09 lemure kernel: [ 381.451217] Code: 1a 00 b8 64 00 00 00 2b 05 5c a4 28 01 be 64 00 00 00 31 d2 8b 7d e0 48 0f af c3 48 f7 f6 31 d2 48 89 c1 48 0f af 4d e8 48 89 c8 <48> f7 f7 31 d2 48 89 c1 41 8b 84 24 4c 01 00 00 48 0f af c3 48
Sep 23 17:41:10 lemure kernel: [ 381.471131] RIP [<ffffffff810e8bc2>] bdi_dirty_limit+0x5a/0x9e
Sep 23 17:41:10 lemure kernel: [ 381.477057] RSP <ffff88041ad03d40>
Sep 23 17:41:10 lemure kernel: [ 381.480604] ---[ end trace 703f173ed75f76a9 ]---
Thanks.
--
Regards/Gruss,
Boris.
Advanced Micro Devices GmbH
Einsteinring 24, 85609 Dornach
GM: Alberto Bozzo
Reg: Dornach, Landkreis Muenchen
HRB Nr. 43632 WEEE Registernr: 129 19551
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