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Date:   Sun, 23 Aug 2020 00:29:20 +0800
From:   Guohua Zhong <zhongguohua1@...wei.com>
To:     <christophe.leroy@...roup.eu>
CC:     <benh@...nel.crashing.org>, <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        <mpe@...erman.id.au>, <nixiaoming@...wei.com>, <paulus@...ba.org>,
        <stable@...r.kernel.org>, <wangle6@...wei.com>,
        <zhongguohua1@...wei.com>
Subject: Re: [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero

>> 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@...wei.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@...r.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

>You don't need to load r9 with 0, use cmplwi instead.

I will change cmplw to cmplwi in next patch as your suggestion. Thanks

>> +	cmplw	r4,r9	# check if divisor r4 is zero
>> +	beq	5f			# jump to label 5 if r4(divisor) is zero

>You should leave space between the compare and the branch (i.e. have 
>other instructions inbetween when possible), so that the processor can 
>prepare the branching and do a good prediction. Same as the compare 
>below, you see that there are two other instructions between the cmplw 
>are the blt. You can eventually use another cr field than cr0 in order 
>to nest several test/branches.
>Also because on recent powerpc32, instructions are fetched and executed 
>two by two.

Good advice! 

OK, let two lwz instructions between campare and branch as below:

diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S
index 3d5426e7dcc4..570774d9782d 100644
        li      r8,0
--- a/arch/powerpc/lib/div64.S
+++ b/arch/powerpc/lib/div64.S
@@ -13,8 +13,10 @@
 #include <asm/processor.h>

 _GLOBAL(__div64_32)
+ cmplwi      r4,0    # check if divisor r4 is zero
        lwz     r5,0(r3)        # get the dividend into r5/r6
        lwz     r6,4(r3)
+ beq 5f                      # jump to label 5 if r4(divisor) is zero
        cmplw   r5,r4
        li      r7,0
        li      r8,0
lines 97-110
 4:     stw     r7,0(r3)        # return the quotient in *r3
        stw     r8,4(r3)
        mr      r3,r6           # return the remainder in r3
-   blr
+5:   blr                             # return if divisor r4 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
>> 

Guohua

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