RFR (S) [Graal] runtime/CommandLine/PrintTouchedMethods.java crashes with assertion "reference count underflow for symbol"

Ioi Lam ioi.lam at oracle.com
Mon Mar 26 05:18:34 UTC 2018

On 3/25/18 4:28 PM, David Holmes wrote:
> On 26/03/2018 1:23 AM, coleen.phillimore at oracle.com wrote:
>> On 3/25/18 9:08 AM, David Holmes wrote:
>>> Hi Ioi,
>>> On 24/03/2018 7:38 AM, Ioi Lam wrote:
>>>> https://bugs.openjdk.java.net/browse/JDK-8199793
>>>> http://cr.openjdk.java.net/~iklam/jdk11/8199793-PrintTouchedMethods-crash.v01/ 
>>>> The crash is in
>>>>      V [libjvm.so+0x16fe226] Symbol::decrement_refcount()+0xe6
>>>>      V [libjvm.so+0x1026e0b] JVM_FindLoadedClass+0x20b
>>>> and the log file says "Symbol: 'java/lang/invoke/LambdaForm$BMH' 
>>>> count -1".
>>>> This seems to be a race condition between Symbol::decrement_refcount()
>>>> vs Symbol::set_permanent(). The former uses an atomic increment and 
>>>> is called by
>>>> JVM_FindLoadedClass. The latter does a simple store of a short 
>>>> value of -1, and is
>>>> called only by Method::log_touched() when -XX:+LogTouchedMethods is 
>>>> enabled.
>>>> Apparently we have a Symbol whose refcount started with a positive 
>>>> value.
>>>> While one thread is calling Symbol::decrement_refcount() and a second
>>>> thread calls Symbol::set_permanent() at the same time, the 
>>>> unexpected value -1
>>>> could be returned to the first thread.
>>>> FIX:
>>>> I changed Method::log_touched() to use Symbol::increment_refcount 
>>>> instead.
>>>> I can no longer reproduce the crash after this change.
>>>> Also, because the behavior of Symbol::set_permanent is not well 
>>>> understood
>>>> and has shown to be racy, I removed this function altogether.
>>> So when will a Symbol now ever be "permanent"? Isn't all the code 
>>> related to is_permanent() and PERM_REFCOUNT dead now? And the checks 
>>> for _refcount >= 0 ?
>> When we create a symbol, it can have a permanent refcount.
> Okay so now there are two types of "permanent" symbols - which seems 
> confusing. Those set at creation time, and those previously set 
> through set_permanent which now simply bump the refcount. And you can 
> no longer identify the second set as being permanent.
After my patch, there will be only one type of permanent symbol -- when 
you create a symbol, you know it will be alive forever (e.g., a symbol 
requested by the null class loader).

I am not quite sure about this history of why this is necessary -- 
except for symbols in the CDS archive. Those are marked as permanent so 
you won't change its refcount (because it's read only).

Perhaps it's for performance reasons? There are a lot of Symbols created 
by the boot loader and are referenced by many classes. So if we don't 
need to atomically modify their _refcounts then it pays off??

Or, maybe it's for memory saving -- allocating from arena has less 
overhead than malloc??

Anyway, regardless of whether a symbol is permanent or not, the usual 
rule is -- when you store a symbol (e.g., as an UTF8 into a 
ConstantPool), you increment its refcount. When you release a symbol 
(e.g., when said ConstantPool is deallocated), you decrement its refcount.

The only exception to this rule was Method::log_touched(), which used 
set_permanent instead. It stores a symbol (in its hashtable) but never 
releases it (the hashtable persists for the entire VM run time). So in 
this case, it's perfectly OK to call increment_reference instead.

>>> The other way to avoid the race is to implement inc/dec using CAS 
>>> instead of raw atomic add/sub.
>> That seems expensive.
> If you wanted to keep the existing notion of permanent and how it is 
> recorded, then it is a way to do that.

I think removing set_permanent will make the code cleaner and easier to 

- Ioi
> David
>> Coleen
>>> Thanks,
>>> David
>>>> Thanks
>>>> - Ioi

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