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

[Tobias Hartmann]

Hi Ioi,

this looks good to me too. Thanks for investigating!

Best regards,
Tobias

On 23.03.2018 22:38, 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