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SubjectRe: [BUG REPORT] ktime_get_ts64 causes Hard Lockup
From
output from objdump provided:

static __always_inline u32
__iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
{
u32 ret = 0;
27b: 31 d2 xor %edx,%edx
while (dividend >= divisor) {
/* The following asm() prevents the compiler from
optimising this loop into a modulo operation. */
asm("" : "+rm"(dividend));

dividend -= divisor;
27d: 48 2d 00 ca 9a 3b sub $0x3b9aca00,%rax
ret++;
283: 83 c2 01 add $0x1,%edx
static __always_inline u32
__iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
{
u32 ret = 0;

while (dividend >= divisor) {
286: 48 3d ff c9 9a 3b cmp $0x3b9ac9ff,%rax
28c: 77 ef ja 27d <ktime_get_ts64+0x9d>
* This must always be inlined because its used from the x86-64 vdso,
* which cannot call other kernel functions.
*/

I guess is dividend and divsor get out of whack this function loops
forever. So I sent this to the maintainers, so which is the list of
"engineers" who actually wrote and understand this section of code. I
would like to get this fixed.

Thanks

Jeff

On 1/18/16, Jeff Merkey <linux.mdb@gmail.com> wrote:
> On 1/18/16, Jeff Merkey <linux.mdb@gmail.com> wrote:
>> If I suspend all processors in the NMI handler while in the debugger
>> console at an active breakpoint, and then I leave the system in the
>> console for about 20 minutes with the processors spinning, then exit
>> and release the processors, ktime_get_ts64 enters an infinite loop and
>> triggers the Hard Lockup detector. Nice having a debugger to just
>> step into the NMI handlers and find this stuff now.
>>
>> The offending code is:
>>
>> (2)> u ktime_get_ts64+9d
>> <<< this section gets stuck in an infinite loop.
>> 0xffffffff810ede1d 482D00CA9A3B sub rax,0x3b9aca00
>> 0xffffffff810ede23 83C201 add edx,0x1
>> 0xffffffff810ede26 483DFFC99A3B cmp rax,0x3b9ac9ff
>> 0xffffffff810ede2c 77EF ja ktime_get_ts64+0x9d
>> (0xffffffff810ede1d) (up)
>> <<<
>> 0xffffffff810ede2e 4801CA add rdx,rcx
>> 0xffffffff810ede31 48894308 mov QWORD PTR
>> [rbx+8]=0xFFFFFFFFA0774E01,rax
>> 0xffffffff810ede35 488913 mov QWORD PTR
>> [rbx]=0xFFFFFFFFA0742000,rdx
>> 0xffffffff810ede38 5B pop rbx
>> 0xffffffff810ede39 415C pop r12
>> 0xffffffff810ede3b 415D pop r13
>> 0xffffffff810ede3d 5D pop rbp
>> 0xffffffff810ede3e C3 ret
>> 0xffffffff810ede3f 31D2 xor edx,edx
>> 0xffffffff810ede41 EBEB jmp ktime_get_ts64+0xae
>> (0xffffffff810ede2e) (up)
>> 0xffffffff810ede43 BE11030000 mov esi,0x311
>> 0xffffffff810ede48 48C7C751C38D81 mov rdi,0xffffffff818dc351
>> 0xffffffff810ede4f E82C12F9FF call warn_slowpath_null
>> 0xffffffff810ede54 E946FFFFFF jmp ktime_get_ts64+0x1f
>> (0xffffffff810edd9f) (up)
>> 0xffffffff810ede59 F390 pause
>> 0xffffffff810ede5b E942FFFFFF jmp ktime_get_ts64+0x22
>> (0xffffffff810edda2) (up)
>>
>> Same code in GDB format
>>
>> (2)> id ktime_get_ts64+9d
>> <<<
>> 0xffffffff810ede1d ktime_get_ts64+0x9d: sub $0x3b9aca00,%rax
>> 0xffffffff810ede23 ktime_get_ts64+0xa3: add $0x1,%edx
>> 0xffffffff810ede26 ktime_get_ts64+0xa6: cmp $0x3b9ac9ff,%rax
>> 0xffffffff810ede2c ktime_get_ts64+0xac: ja 0xffffffff810ede1d
>> ktime_get_ts64+0x9d (up)
>> <<<
>> 0xffffffff810ede2e ktime_get_ts64+0xae: add %rcx,%rdx
>> 0xffffffff810ede31 ktime_get_ts64+0xb1: mov %rax,0x8(%rbx)
>> 0xffffffff810ede35 ktime_get_ts64+0xb5: mov %rdx,(%rbx)
>> 0xffffffff810ede38 ktime_get_ts64+0xb8: pop %rbx
>> 0xffffffff810ede39 ktime_get_ts64+0xb9: pop %r12
>> 0xffffffff810ede3b ktime_get_ts64+0xbb: pop %r13
>> 0xffffffff810ede3d ktime_get_ts64+0xbd: pop %rbp
>> 0xffffffff810ede3e ktime_get_ts64+0xbe: retq
>> 0xffffffff810ede3f ktime_get_ts64+0xbf: xor %edx,%edx
>> 0xffffffff810ede41 ktime_get_ts64+0xc1: jmp 0xffffffff810ede2e
>> ktime_get_ts64+0xae (up)
>> 0xffffffff810ede43 ktime_get_ts64+0xc3: mov $0x311,%esi
>> 0xffffffff810ede48 ktime_get_ts64+0xc8: mov
>> $0xffffffff818dc351,%rdi
>> 0xffffffff810ede4f ktime_get_ts64+0xcf: callq 0xffffffff8107f080
>> warn_slowpath_null
>> 0xffffffff810ede54 ktime_get_ts64+0xd4: jmpq 0xffffffff810edd9f
>> ktime_get_ts64+0x1f (up)
>> 0xffffffff810ede59 ktime_get_ts64+0xd9: pause
>> 0xffffffff810ede5b ktime_get_ts64+0xdb: jmpq 0xffffffff810edda2
>> ktime_get_ts64+0x22 (up)
>> (2)> g
>>
>>
>> What is strange is the math its doing. It is subtracting a fixed
>> value from rax then comparing the value. It looks like this is case
>> where the value may have wrapped and the code just wasn;t setup to
>> handle it.
>>
>> 0xffffffff810ede1d 482D00CA9A3B sub rax,0x3b9aca00
>> 0xffffffff810ede23 83C201 add edx,0x1
>> 0xffffffff810ede26 483DFFC99A3B cmp rax,0x3b9ac9ff
>> 0xffffffff810ede2c 77EF ja ktime_get_ts64+0x9d
>> (0xffffffff810ede1d) (up)
>>
>> The C code is:
>>
>>
>> /**
>> * ktime_get_ts64 - get the monotonic clock in timespec64 format
>> * @ts: pointer to timespec variable
>> *
>> * The function calculates the monotonic clock from the realtime
>> * clock and the wall_to_monotonic offset and stores the result
>> * in normalized timespec64 format in the variable pointed to by @ts.
>> */
>> void ktime_get_ts64(struct timespec64 *ts)
>> {
>> struct timekeeper *tk = &tk_core.timekeeper;
>> struct timespec64 tomono;
>> s64 nsec;
>> unsigned int seq;
>>
>> WARN_ON(timekeeping_suspended);
>>
>> do {
>> seq = read_seqcount_begin(&tk_core.seq);
>> ts->tv_sec = tk->xtime_sec;
>> nsec = timekeeping_get_ns(&tk->tkr_mono);
>> tomono = tk->wall_to_monotonic;
>> <<<
>> } while (read_seqcount_retry(&tk_core.seq, seq));
>> <<<
>> ts->tv_sec += tomono.tv_sec;
>> ts->tv_nsec = 0;
>> timespec64_add_ns(ts, nsec + tomono.tv_nsec);
>> }
>> EXPORT_SYMBOL_GPL(ktime_get_ts64);
>>
>> Any ideas how to fix this problem? That do {} while gets stuck there.
>>
>> Jeff
>>
>
> I just verified that both kgdb and kdb trigger this bug as well if you
> hold the processors suspended for about 20-45 minutes.
>
> Jeff
>

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