On 1/23/16, Jeff Merkey <linux.mdb@xxxxxxxxx> wrote:
> If I leave the system in the debugger console overnight with all the
> processors suspended for about 8 hours, then type go, the following
> bug shows up during file I/O. This particular bug showed up while
> using git to update some branches.
>
> I have only seen this bug once and I attempted to reproduce it to get
> a trace dump but have not been able to trigger it again. The NULL
> pointer is RDI set to NULL while trying to obtain a lock.
>
> (2)> .z grab_journal
> ffffffffa00bb740 t jbd2_journal_grab_journal_head [jbd2]
> (2)> u ffffffffa00bb740
> jbd2|jbd2_journal_grab_journal_head:
> 0xffffffffa00bb740 0F1F440000 nop DWORD PTR [rax+rax]=0x0
> 0xffffffffa00bb745 55 push rbp
> 0xffffffffa00bb746 4889E5 mov rbp,rsp
> <<<<<<<<<<<< Crashes here with RDI set to NULL
> 0xffffffffa00bb749 F00FBA2F18 lock bts DWORD PTR [rdi]=0x0,0x18
> <<<<<<<<<<<<
> 0xffffffffa00bb74e 7219 jb
> jbd2_journal_grab_journal_head+0x29 (0xffffffffa00bb769) (down)
> 0xffffffffa00bb750 488B07 mov rax,QWORD PTR [rdi]=0x0
> 0xffffffffa00bb753 A900000200 test eax,0x20000
> 0xffffffffa00bb758 741D je
> jbd2_journal_grab_journal_head+0x37 (0xffffffffa00bb777) (down)
> 0xffffffffa00bb75a 488B4740 mov rax,QWORD PTR [rdi+64]=0x0
> 0xffffffffa00bb75e 83400801 add DWORD PTR [rax+8]=0x0,0x1
> 0xffffffffa00bb762 F0806703FE lock and BYTE PTR [rdi+3]=0x00,0xfe
> 0xffffffffa00bb767 5D pop rbp
> 0xffffffffa00bb768 C3 ret
> 0xffffffffa00bb769 F390 pause
> 0xffffffffa00bb76b 488B07 mov rax,QWORD PTR [rdi]=0x0
> 0xffffffffa00bb76e A900000001 test eax,0x1000000
> 0xffffffffa00bb773 75F4 jne
> jbd2_journal_grab_journal_head+0x29 (0xffffffffa00bb769) (up)
> 0xffffffffa00bb775 EBD2 jmp
> jbd2_journal_grab_journal_head+0x9 (0xffffffffa00bb749) (up)
> 0xffffffffa00bb777 31C0 xor eax,eax
>
> The backtrace showed this function being called from the swapper
> thread when the crash occurred. It's damn hard to reproduce. If I
> see it again, I'll get you a better trace.
>
> Jeff
>
Here is an objdump of the code in question.
0000000000005540 <jbd2_journal_grab_journal_head>:
/*
* Grab a ref against this buffer_head's journal_head. If it ended up not
* having a journal_head, return NULL
*/
struct journal_head *jbd2_journal_grab_journal_head(struct buffer_head *bh)
{
5540: e8 00 00 00 00 callq 5545 <jbd2_journal_grab_journal_head+0x5>
5545: 55 push %rbp
5546: 48 89 e5 mov %rsp,%rbp
* This operation is atomic and cannot be reordered.
* It also implies a memory barrier.
*/
static inline int test_and_set_bit(long nr, volatile unsigned long *addr)
{
GEN_BINARY_RMWcc(LOCK_PREFIX "bts", *addr, "Ir", nr, "%0", "c");
5549: f0 0f ba 2f 18 lock btsl $0x18,(%rdi) << crashes here RDI=NULL
554e: 72 19 jb 5569
<jbd2_journal_grab_journal_head+0x29>
}
static __always_inline int constant_test_bit(long nr, const volatile
unsigned long *addr)
{
return ((1UL << (nr & (BITS_PER_LONG-1))) &
(addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
5550: 48 8b 07 mov (%rdi),%rax
struct journal_head *jh = NULL;
jbd_lock_bh_journal_head(bh);
if (buffer_jbd(bh)) {
5553: a9 00 00 02 00 test $0x20000,%eax
5558: 74 1d je 5577
<jbd2_journal_grab_journal_head+0x37>
/*
* Grab a ref against this buffer_head's journal_head. If it ended up not
* having a journal_head, return NULL
*/
struct journal_head *jbd2_journal_grab_journal_head(struct buffer_head *bh)
555a: 48 8b 47 40 mov 0x40(%rdi),%rax
struct journal_head *jh = NULL;
jbd_lock_bh_journal_head(bh);
if (buffer_jbd(bh)) {
jh = bh2jh(bh);
jh->b_jcount++;
555e: 83 40 08 01 addl $0x1,0x8(%rax)
*/
static __always_inline void
clear_bit(long nr, volatile unsigned long *addr)
{
if (IS_IMMEDIATE(nr)) {
asm volatile(LOCK_PREFIX "andb %1,%0"
5562: f0 80 67 03 fe lock andb $0xfe,0x3(%rdi)
}
jbd_unlock_bh_journal_head(bh);
return jh;
}
5567: 5d pop %rbp
5568: c3 retq
5569: f3 90 pause
}
static __always_inline int constant_test_bit(long nr, const volatile
unsigned long *addr)
{
return ((1UL << (nr & (BITS_PER_LONG-1))) &
(addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
556b: 48 8b 07 mov (%rdi),%rax
556e: a9 00 00 00 01 test $0x1000000,%eax
5573: 75 f4 jne 5569
<jbd2_journal_grab_journal_head+0x29>
preempt_disable();
5575: eb d2 jmp 5549 <jbd2_journal_grab_journal_head+0x9>
* Grab a ref against this buffer_head's journal_head. If it ended up not
* having a journal_head, return NULL
*/
struct journal_head *jbd2_journal_grab_journal_head(struct buffer_head *bh)
{
struct journal_head *jh = NULL;
5577: 31 c0 xor %eax,%eax
5579: eb e7 jmp 5562
<jbd2_journal_grab_journal_head+0x22>
557b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
0000000000005580 <jbd2_journal_put_journal_head>:
/*
* Drop a reference on the passed journal_head. If it fell to zero then
* release the journal_head from the buffer_head.
*/
void jbd2_journal_put_journal_head(struct journal_head *jh)
{
5580: e8 00 00 00 00 callq 5585 <jbd2_journal_put_journal_head+0x5>
5585: 55 push %rbp
5586: 48 89 e5 mov %rsp,%rbp
5589: 41 54 push %r12
558b: 53 push %rbx
/*
* Drop a reference on the passed journal_head. If it fell to zero then
* release the journal_head from the buffer_head.
*/
void jbd2_journal_put_journal_head(struct journal_head *jh)
558c: 48 8b 1f mov (%rdi),%rbx
* This operation is atomic and cannot be reordered.
* It also implies a memory barrier.
*/
static inline int test_and_set_bit(long nr, volatile unsigned long *addr)
{
GEN_BINARY_RMWcc(LOCK_PREFIX "bts", *addr, "Ir", nr, "%0", "c");
558f: f0 0f ba 2b 18 lock btsl $0x18,(%rbx)
5594: 72 1f jb 55b5 <jbd2_journal_put_journal_head+0x35>
{
struct buffer_head *bh = jh2bh(jh);
jbd_lock_bh_journal_head(bh);
J_ASSERT_JH(jh, jh->b_jcount > 0);
5596: 8b 47 08 mov 0x8(%rdi),%eax
5599: 85 c0 test %eax,%eax
559b: 0f 8e be 00 00 00 jle 565f <jbd2_journal_put_journal_head+0xdf>
--jh->b_jcount;
55a1: 83 e8 01 sub $0x1,%eax
if (!jh->b_jcount) {
55a4: 85 c0 test %eax,%eax
{
struct buffer_head *bh = jh2bh(jh);
jbd_lock_bh_journal_head(bh);
J_ASSERT_JH(jh, jh->b_jcount > 0);
--jh->b_jcount;
55a6: 89 47 08 mov %eax,0x8(%rdi)
if (!jh->b_jcount) {
--
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