linux-kernel - Re: [BUG REPORT] ktime_get

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Message-ID: <CAO6TR8UjFQbWJ32u12axm8B54KK6i63=xOikgrVUq9-W_dmjEw@mail.gmail.com>
Date:	Mon, 18 Jan 2016 19:16:45 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup

On 1/18/16, Jeff Merkey <linux.mdb@...il.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