RFR: 8011760 assert(delta != 0) failed: dup pointer in MemBaseline::malloc_sort_by_addr

[Zhengyu Gu]

Hi David,

On 7/10/2013 12:45 AM, David Holmes wrote:
> On 9/07/2013 3:23 AM, Zhengyu Gu wrote:
>> This is a simple fix to qsort's comparison function. The bug is 
>> related to libc version, it only happens on Linux 32 bit with certain 
>> versions of libc. The one I used to reproduce, is libc-2.11.1.so.
>>
>> Most of qsort algorithms do not compare element to itself, but 
>> apparently it is not the case with certain version of Linux 32 libc 
>> implementation.
>
> Sorry but I don't get it. You are asserting that the 
> pointers/addresses should be distinct but you skip the assert if they 
> are equal ???
>
>   int MemBaseline::malloc_sort_by_addr(const void* p1, const void* p2) {
>     assert(MemTracker::is_on(), "Just check");
>     const MemPointerRecord* mp1 = (const MemPointerRecord*)p1;
>     const MemPointerRecord* mp2 = (const MemPointerRecord*)p2;
>     int delta = UNSIGNED_COMPARE(mp1->addr(), mp2->addr());
> !   assert(p1 == p2 || delta != 0, "dup pointer");
>     return delta;
>   }
>
> And is there some case where delta==0 but p1 != p2 ?
>
No, it should not. The snapshot contains "live" malloc records, which 
memory block addresses have to be unique.

This is the comparison function passed to qsort, most qsort 
implementations do not compare an element to itself, so there will not 
be p1 == p2. But on Linux x86 with some versions of libc, there are 
exceptions.


p1 and p2 are NOT pointers to real malloc addresses, they are pointers 
to malloc bookkeeping record MemPointerRecord. mp1->addr() and 
mp2->addr() are the real malloc addresses.

Thanks,

-Zhengyu

> Thanks,
> David
>
>>
>> JBS:   https://jbs.oracle.com/bugs/browse/JDK-8011760
>> Public bug: not available
>> Webrev:  http://cr.openjdk.java.net/~zgu/8011760/webrev.00/
>>
>>
>> Thanks,
>>
>> -Zhengyu
>>