RFR (XS): 7129715: MAC: SIGBUS in nsk stress test (original) (raw)

Roland Westrelin roland.westrelin at oracle.com
Fri Jun 15 03:20:54 PDT 2012

Hi Dan,

Thanks for taking a look at this. See below.

Thanks for tackling such nasty code... Just trying to understand this one... These checks:

476 if (sig == SIGSEGV || sig == SIGBUS) { 480 if (addr < thread->stackbase() && 481 addr >= thread->stackbase() - thread->stacksize()) { 483 if (thread->instackyellowzone(addr)) { 485 if (thread->threadstate() == threadinJava) { tell us that we took a SIGSEGV or SIGBUS while running Java code in the yellow zone of our stack... so stack overflow... which gets us to this setting of "stub": 488 stub = SharedRuntime::continuationforimplicitexception(thread, pc, SharedRuntime::STACKOVERFLOW);

This line: 519 if (thread->threadstate() == threadinJava) { gets us into another block of "stub" setting code, but it currently doesn't care that "stub" was already set. That's the code you're trying to fix with this new line: 519 if (thread->threadstate() == threadinJava && stub == NULL) {

Exactly.

Just to be complete, I'm trying to understand which of the many places that set "stub" is clobbering the existing value.

This is the code that clobbers the stub value:

530 } else if (sig == SIGBUS && MacroAssembler::needs_explicit_null_check((intptr_t)info->si_addr)) { 534 // BugId 4454115: A read from a MappedByteBuffer can fault 535 // here if the underlying file has been truncated. 536 // Do not crash the VM in such a case. 537 CodeBlob* cb = CodeCache::find_blob_unsafe(pc); 538 nmethod* nm = cb->is_nmethod() ? (nmethod*)cb : NULL; 539 if (nm != NULL && nm->has_unsafe_access()) { 540 stub = StubRoutines::handler_for_unsafe_access(); 541 } 542 }

si_addr is an address on the stack so it's not in the first page and MacroAssembler::needs_explicit_null_check() returns true. The method where the SIGBUS due to the stack overflow happens is a compiled method and we're unlucky because it has some unsafe accesses and the SIGBUS is mistaken for an unsafe access that has gone wrong.

Roland.

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