On Tue, 1 Jul 2025 16:01:10 GMT, Leonid Mesnik <lmes...@openjdk.org> wrote:
>> The segv/eav happens in the case if JvmtiBreakpoint::_method's class
>> redefined old between getting the Method* from jmethodid in the
>> JvmtiEnv::SetBreakpoint(Method* method, jlocation location) {..} and
>> and actual setting breakpoint in the VM operation VM_ChangeBreakpoints.
>>
>> Here are details:
>> The breakpoint is set in 2 steps.
>> 1) method jvmti_SetBreakpoint(jvmtiEnv* env, jmethodID method, jlocation
>> location) convert jmethodID to Method* and call
>> JvmtiEnv::SetBreakpoint(Method* method, jlocation location)
>> where
>> JvmtiBreakpoint bp(method, location);
>> is created with this Method*
>> Note: it is done while thread is in VM state, so Method can't become is_old
>> while this is done.
>>
>> 2) The VMOp is used to add breakpoint into the list
>> VM_ChangeBreakpoints set_breakpoint(VM_ChangeBreakpoints::SET_BREAKPOINT,
>> &bp);
>> VMThread::execute(&set_breakpoint);
>> to call JvmtiBreakpoints::set_at_safepoint()
>> that can modify JvmtiBreakpoints list and set breakpoint in safepoint
>> without synchronization.
>>
>> So it might be possible that class redefinition VM_RedefineClasses
>> operation that redefine the class with this breakpoint happens between steps
>> 1) and 2)
>> VM_RedefineClasses::redefine_single_class()
>> clear all class-related breakpoints in the JvmtiBreakpoints, however the
>> "problematic" breakpoint is in VMThread queue and thus we are still
>> continue to do this operation.
>> So in the step 2) the the JvmtiBreakpoint with 'is_old' method is added to
>> the JvmtiBreakpoints and breakpoint is set.
>>
>> Then old method mights be purged any time once they are not on the stack
>> and any access to this breakpoint could lead to usage of Metthod* _method
>> pointing to deallocated metaspace.
>>
>> The VM_RedefineClasses clear all breakpoints so it is correct just to don't
>> proceed with current breakpoint also.
>>
>> Looks, like very unlikely but reproducing with stress test after some time.
>> Verified that the crash is not reproduced anymore with corresponding test
>> after the fix.
>>
>> Many thanks to Coleen for detailed explanation of class redefinition.
>
> Leonid Mesnik has updated the pull request incrementally with two additional
> commits since the last revision:
>
> - Update src/hotspot/share/prims/jvmtiImpl.cpp
>
> Co-authored-by: Coleen Phillimore <coleen.phillim...@oracle.com>
> - Update src/hotspot/share/prims/jvmtiImpl.cpp
>
> Co-authored-by: Coleen Phillimore <coleen.phillim...@oracle.com>
Looks good.
-------------
Marked as reviewed by coleenp (Reviewer).
PR Review: https://git.openjdk.org/jdk/pull/26031#pullrequestreview-2976297700
