When cat /proc/pid/stat, do_task_stat will call into cputime_adjust, which call stack is like this:
[17179954.674326]BookE Watchdog detected hard LOCKUP on cpu 0 [17179954.674331]dCPU: 0 PID: 1262 Comm: TICK Tainted: P W O 4.4.176 #1 [17179954.674339]dtask: dc9d7040 task.stack: d3cb4000 [17179954.674344]NIP: c001b1a8 LR: c006a7ac CTR: 00000000 [17179954.674349]REGS: e6fe1f10 TRAP: 3202 Tainted: P W O (4.4.176) [17179954.674355]MSR: 00021002 <CE,ME> CR: 28002224 XER: 00000000 [17179954.674364] GPR00: 00000016 d3cb5cb0 dc9d7040 d3cb5cc0 00000000 0000025d ffe15b24 ffffffff GPR08: de86aead 00000000 000003ff ffffffff 28002222 0084d1c0 00000000 ffffffff GPR16: b5929ca0 b4bb7a48 c0863c08 0000048d 00000062 00000062 00000000 0000000f GPR24: 00000000 d3cb5d08 d3cb5d60 d3cb5d64 00029002 d3e9c214 fffff30e d3e9c20c [17179954.674410]NIP [c001b1a8] __div64_32+0x60/0xa0 [17179954.674422]LR [c006a7ac] cputime_adjust+0x124/0x138 [17179954.674434]Call Trace: [17179961.832693]Call Trace: [17179961.832695][d3cb5cb0] [c006a6dc] cputime_adjust+0x54/0x138 (unreliable) [17179961.832705][d3cb5cf0] [c006a818] task_cputime_adjusted+0x58/0x80 [17179961.832713][d3cb5d20] [c01dab44] do_task_stat+0x298/0x870 [17179961.832720][d3cb5de0] [c01d4948] proc_single_show+0x60/0xa4 [17179961.832728][d3cb5e10] [c01963d8] seq_read+0x2d8/0x52c [17179961.832736][d3cb5e80] [c01702fc] __vfs_read+0x40/0x114 [17179961.832744][d3cb5ef0] [c0170b1c] vfs_read+0x9c/0x10c [17179961.832751][d3cb5f10] [c0171440] SyS_read+0x68/0xc4 [17179961.832759][d3cb5f40] [c0010a40] ret_from_syscall+0x0/0x3c
do_task_stat->task_cputime_adjusted->cputime_adjust->scale_stime->div_u64 ->div_u64_rem->do_div->__div64_32
In some corner case, stime + utime = 0 if overflow. Even in v5.8.2 kernel the cputime has changed from unsigned long to u64 data type. About 200 days, the lowwer 32 bit will be 0x00000000. Because divisor for __div64_32 is unsigned long data type,which is 32 bit for powepc 32, the bug still exists.
So it is also a bug in the cputime_adjust which does not check if stime + utime = 0
time = scale_stime((__force u64)stime, (__force u64)rtime, (__force u64)(stime + utime));
The commit 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()") in mainline kernel may has fixed this case. But it is also better to check if divisor is 0 in __div64_32 for other situation.
Signed-off-by: Guohua Zhong zhongguohua1@huawei.com Fixes:14cf11af6cf6 "( powerpc: Merge enough to start building in arch/powerpc.)" Fixes:94b212c29f68 "( powerpc: Move ppc64 boot wrapper code over to arch/powerpc)" Cc: stable@vger.kernel.org # v2.6.15+ --- arch/powerpc/boot/div64.S | 4 ++++ arch/powerpc/lib/div64.S | 4 ++++ 2 files changed, 8 insertions(+)
diff --git a/arch/powerpc/boot/div64.S b/arch/powerpc/boot/div64.S index 4354928ed62e..39a25b9712d1 100644 --- a/arch/powerpc/boot/div64.S +++ b/arch/powerpc/boot/div64.S @@ -13,6 +13,9 @@
.globl __div64_32 __div64_32: + li r9,0 + cmplw r4,r9 # check if divisor r4 is zero + beq 5f # jump to label 5 if r4(divisor) is zero lwz r5,0(r3) # get the dividend into r5/r6 lwz r6,4(r3) cmplw r5,r4 @@ -52,6 +55,7 @@ __div64_32: 4: stw r7,0(r3) # return the quotient in *r3 stw r8,4(r3) mr r3,r6 # return the remainder in r3 +5: # return if divisor r4 is zero blr
/* diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S index 3d5426e7dcc4..1cc9bcabf678 100644 --- a/arch/powerpc/lib/div64.S +++ b/arch/powerpc/lib/div64.S @@ -13,6 +13,9 @@ #include <asm/processor.h>
_GLOBAL(__div64_32) + li r9,0 + cmplw r4,r9 # check if divisor r4 is zero + beq 5f # jump to label 5 if r4(divisor) is zero lwz r5,0(r3) # get the dividend into r5/r6 lwz r6,4(r3) cmplw r5,r4 @@ -52,4 +55,5 @@ _GLOBAL(__div64_32) 4: stw r7,0(r3) # return the quotient in *r3 stw r8,4(r3) mr r3,r6 # return the remainder in r3 +5: # return if divisor r4 is zero blr