linux-kernel - Re: [PATCH] sched/fair: Use 'unsigned long' for group_shares,group

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Date:   Mon, 15 Apr 2019 23:20:31 +0800
From:   "chengjian (D)" <cj.chengjian@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <huawei.libin@...wei.com>, <xiexiuqi@...wei.com>,
        <yangyingliang@...wei.com>, <mingo@...hat.com>,
        <linux-kernel@...r.kernel.org>,
        "chengjian (D)" <cj.chengjian@...wei.com>,
        Yang Yingliang <yangyingliang@...wei.com>
Subject: Re: [PATCH] sched/fair: Use 'unsigned long' for
 group_shares,group_runnable

Hi, Peter


On 2019/4/15 20:46, Peter Zijlstra wrote:

I write a demo about this, which I described it as overflow.

#cat test.c

//test.c
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <assert.h>

int main(void)
{
     long a = 1048576 * 9144968455305; /* shares = tg_shares * load */
     unsigned long b = a;
     //unsigned long e = 1048576 * 9144968455305;

     printf("LONG_MAX = %ld, 0x%lx\n", LONG_MAX, LONG_MAX);
     printf("ULONG_MAX = %lu, 0x%lx\n", ULONG_MAX, ULONG_MAX);

     if (b > LONG_MAX)
         printf("==overflow!!!==\n");   // OVERFLOW
     printf("a = %20ld,0x%016lx\n", a, a);
     printf("b = %20lu,0x%016lx\n", b, b);

     /* shares /= tg_weight */
     printf("a/3 = 0x%016lx\n", (a / 3));                        // WRONG
     printf("a/3 = 0x%016lx\n", (((unsigned long)a) / 3));   // unsigned
     printf("a/3 = 0x%016lx\n", (a / (unsigned long)3));    // unsigned
     printf("b/3 = 0x%016lx\n", b / 3);            // unsigned

     return EXIT_SUCCESS;
}



#./test

LONG_MAX = 9223372036854775807, 0x7fffffffffffffff
ULONG_MAX = 18446744073709551615, 0xffffffffffffffff
==overflow!!!==
a = -8857549630719655936,0x8513a98a48900000
b =  9589194442989895680,0x8513a98a48900000
a/3 = 0xd7068dd8c2daaaab     //  WRONG
a/3 = 0x2c5be32e18300000
a/3 = 0x2c5be32e18300000
b/3 = 0x2c5be32e18300000



> On Sat, Apr 13, 2019 at 03:32:34AM +0000, Cheng Jian wrote:
>> group_share and group_runnable are tracked as 'unsigned long',
>> however some functions using them as 'long' which is ultimately
>> assigned back to 'unsigned long' variables in reweight_entity.
>>
>> Since there is not scope on using a different and signed type,
>> this change improves code consistency and avoids further type
>> conversions. More important, to prevent undefined behavior
>> caused by overflow.
> There is no undefined behaviour due to overflow.UBSAN is broken,
> upgrade to GCC8 or later.
>
> .


So, function calc_group_shares will return the wrong value.


```cpp

static long calc_group_shares(struct cfs_rq *cfs_rq)
{

     // ......

     shares = (tg_shares * load);
     if (tg_weight)
         shares /= tg_weight;
}

```


The same to calc_group_runnable.


Thanks.

     CHENG Jian.