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

[David Holmes]

On 10/07/2013 9:38 PM, Zhengyu Gu wrote:
> 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.

Okay now I get it. You want the assert to fire if p1 != p2 but 
p1->addr() == p2->addr()

Thanks,
David

>
> 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
>>>
>