The following test failed in the JDK15 CI:

serviceability/jvmti/CompiledMethodLoad/Zombie.java

nmethod::can_convert_to_zombie() used to be called only by the sweeper, after it has checked that an nmethod is not entrant. Due to the single call site, there was an assert checking that the nmethod should be not entrant (the condition just checked).
However, it is now used in nmethod::post_compiled_method_load_event(), where the result of the assert is racy.

Crash looks like this:
Current thread (0x00007faf71810620): JavaThread "Thread-0" daemon [_thread_in_vm, id=6407, stack(0x000070000c38c000,0x000070000c48c000)]

Stack: [0x000070000c38c000,0x000070000c48c000], sp=0x000070000c48b8e0, free space=1022k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.dylib+0xb3ca44] VMError::report_and_die(int, char const*, char const*, __va_list_tag*, Thread*, unsigned char*, void*, void*, char const*, int, unsigned long)+0x670
V [libjvm.dylib+0xb3d16b] VMError::report_and_die(Thread*, void*, char const*, int, char const*, char const*, __va_list_tag*)+0x47
V [libjvm.dylib+0x334190] report_vm_error(char const*, int, char const*, char const*, ...)+0x145
V [libjvm.dylib+0x8e7ced] nmethod::can_convert_to_zombie()+0x47
V [libjvm.dylib+0x8e8e4a] nmethod::post_compiled_method_load_event(JvmtiThreadState*)+0x2a
V [libjvm.dylib+0x70b913] JvmtiCodeBlobEvents::generate_compiled_method_load_events(JvmtiEnv*)+0xdb
V [libjvm.dylib+0x71fb68] jvmti_GenerateEvents(_jvmtiEnv*, jvmtiEvent)+0xf7
C [libCompiledZombie.dylib+0xd35] GenerateEventsThread+0x45
V [libjvm.dylib+0x780110] JvmtiAgentThread::call_start_function()+0x3c
V [libjvm.dylib+0xab2f73] JavaThread::thread_main_inner()+0x193
V [libjvm.dylib+0xaaf6c9] Thread::call_run()+0x11b
V [libjvm.dylib+0x9247ec] thread_native_entry(Thread*)+0xe0
C [libsystem_pthread.dylib+0x3661] _pthread_body+0x154
C [libsystem_pthread.dylib+0x350d] _pthread_body+0x0
C [libsystem_pthread.dylib+0x2bf9] thread_start+0xd