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

[coleen.phillimore at oracle.com]

This looks good.  Thank you for removing the dangerous function, which 
didn't look dangerous until reading your analysis.
Coleen

On 3/23/18 5:38 PM, Ioi Lam wrote:
> https://bugs.openjdk.java.net/browse/JDK-8199793
> http://cr.openjdk.java.net/~iklam/jdk11/8199793-PrintTouchedMethods-crash.v01/ 
>
>
>
> ANALYSIS:
>
> 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.
>
> Thanks
> - Ioi