Hi Serguei, I have prepared a new webrev based on your suggestions.
Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.6/ Delta: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.6.inc/ Thanks, Richard. ______ From: serguei.spit...@oracle.com <serguei.spit...@oracle.com> Sent: Freitag, 21. August 2020 11:22 To: Reingruber, Richard <richard.reingru...@sap.com>; David Holmes <david.hol...@oracle.com>; serviceability-dev@openjdk.java.net Subject: Re: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() Hi Richard, Thank you for the update, it looks really nice. Just several more minor comments though (I hope, the last ones). > Should I rename the variable to spinWaitCycles or something similar? Yes, waitCycles would be better and more consistent. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.5.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java.udiff.html 81 * The wait time is given in cycles. 82 */ 83 public int waitTime; ... 93 waitTime = 1; This line 82 can be removed if you rename waitTime to waitCycles. It is better to initialize waitCycles at definition and remove the line 93. 146 public static void msg(String m) { 147 System.out.println("### Java-Test: " + m); 148 } One of the de-facto standard names for such methods is "log". 80 * Wait time in native, i.e. with walkable stack, after notifying agent thread to do GetLocalObject() call. 89 msg("Test how many frames fit on the stack by performing recursive calls until StackOverflowError is thrown"); Could you, please, reballance the two long lines above? http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.5.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp.frames.html There are several spots that can be simplified a little bit: 95 jvmtiError result; 96 97 result = jvmti->GetErrorName(errCode, &errMsg); ==> jvmtiError result = jvmti->GetErrorName(errCode, &errMsg); The same is true for for the following cases: 115 err = jvmti->RawMonitorEnter(glws_monitor); 125 err = jvmti->RawMonitorExit(glws_monitor); 135 err = jvmti->RawMonitorWait(glws_monitor, 0); 145 err = jvmti->RawMonitorNotify(glws_monitor); 155 err = jvmti->DestroyRawMonitor(glws_monitor); 173 if (errMsg != NULL) { An extra space before NULL. 89 static jvmtiEnv* jvmti_global = NULL; 276 jvmtiEnv* jvmti = jvmti_global; 308 jvmtiEnv* jvmti = jvmti_global; 330 jvmtiEnv* jvmti = jvmti_global; ... 409 jvmtiEnv* jvmti; 419 res = jvm->GetEnv((void **) &jvmti, JVMTI_VERSION_9); 424 jvmti_global = jvmti; Normal practice is to name the "global_jvmti" as "jvmti". Then there is no need to set it at the start of each function. Thanks, Serguei On 8/20/20 23:47, Reingruber, Richard wrote: Hi Serguei, Sorry for the delay in reply and thank you for the update. I like it in general. There are some minor comments though. Excellent, thanks :) I've prepared webrev.5. Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.5/ Delta: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.5.inc/ http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java.frames.html 112 waitTime = (waitTime << 1); // double wait time 113 if (waitTime >= M || waitTime < 0) { 114 waitTime = 1; // reset when too long 115 } The M is too big for time. "waitTime" is roughly the number of cycles spent in a spin wait. M ~= 10^6 cycles does not seem too long. Should I rename the variable to spinWaitCycles or something similar? What about something like this: waitTime = (waitTime << 1) % 32; or waitTime = (waitTime << 1) & 32; I went for // Double wait time, but limit to roughly 10^6 cycles. waitTime = (waitTime << 1) & (M - 1); waitTime = waitTime == 0 ? 1 : waitTime; Masking the waitTime with % 32 is too small. In my experiments with fastdebug builds I got the crash often with a waitTime of 8K on a Linux server and 256K on my Windows notebook. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp.udiff.html // - Wait for the target thread to either start a new test iteration or to +// signal shutdown. A suggestion to replace: "to either start" => "either to start". Ok, done. +// Shutdown is signalled by setting test_state to ShutDown. The agent reacts +// to it by changing test_state to Terminated and then it exits. The second "it" is not needed: "then it exits" => "then exits". Ok, done. +// ... It sets the shared variable test_state +// to TargetInNative and then it uses the glws_monitor to send the The second "it" is not needed. Ok, done. + monitor_enter(jvmti, env, glws_monitor, AT_LINE); + monitor_notify(jvmti, env, glws_monitor, AT_LINE); + monitor_wait(jvmti, env, glws_monitor, AT_LINE); + monitor_exit(jvmti, env, glws_monitor, AT_LINE); + monitor_destroy(jvmti, env, glws_monitor, AT_LINE); There is only one lock. It'd be more simple to directly use it in the called functions and get rid of the parameter. Just a suggestion, it is up to you to decide. Ok, done. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp.frames.html I'd suggest to refactor the lines 213 and 239-257 to a separate function test_GetLocalObject(jvmti, depth). 240 jobject local_val; Better to rename it to local_obj or just obj. Ok, done. There are still problems with the indent. I reformatted the file using 2 space indentation like in other C++ sources. I didn't include the indentation change in the delta webrev. Thanks, Richard. ______________________ From: mailto:serguei.spit...@oracle.com mailto:serguei.spit...@oracle.com Sent: Donnerstag, 20. August 2020 04:42 To: Reingruber, Richard mailto:richard.reingru...@sap.com; David Holmes mailto:david.hol...@oracle.com; mailto:serviceability-dev@openjdk.java.net Subject: Re: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() Hi Richard, Sorry for the delay in reply and thank you for the update. I like it in general. There are some minor comments though. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java.frames.html 112 waitTime = (waitTime << 1); // double wait time 113 if (waitTime >= M || waitTime < 0) { 114 waitTime = 1; // reset when too long 115 } The M is too big for time. What about something like this: waitTime = (waitTime << 1) % 32; or waitTime = (waitTime << 1) & 32; You can choose a better number instead of 32. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp.udiff.html // - Wait for the target thread to either start a new test iteration or to +// signal shutdown. A suggestion to replace: "to either start" => "either to start". +// Shutdown is signalled by setting test_state to ShutDown. The agent reacts +// to it by changing test_state to Terminated and then it exits. The second "it" is not needed: "then it exits" => "then exits". +// ... It sets the shared variable test_state +// to TargetInNative and then it uses the glws_monitor to send the The second "it" is not needed. + monitor_enter(jvmti, env, glws_monitor, AT_LINE); + monitor_notify(jvmti, env, glws_monitor, AT_LINE); + monitor_wait(jvmti, env, glws_monitor, AT_LINE); + monitor_exit(jvmti, env, glws_monitor, AT_LINE); + monitor_destroy(jvmti, env, glws_monitor, AT_LINE); There is only one lock. It'd be more simple to directly use it in the called functions and get rid of the parameter. Just a suggestion, it is up to you to decide. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp.frames.html I'd suggest to refactor the lines 213 and 239-257 to a separate function test_GetLocalObject(jvmti, depth). 240 jobject local_val; Better to rename it to local_obj or just obj. There are still problems with the indent. The indent 4 is mostly used. However there are still fragments with the indent 2: 112 static void monitor_enter(jvmtiEnv* jvmti, JNIEnv* env, jrawMonitorID mon, const char* loc) { 113 jvmtiError err; 114 115 err = jvmti->RawMonitorEnter(mon); 116 if (err != JVMTI_ERROR_NONE) { 117 ShowErrorMessage(jvmti, err, loc); 118 env->FatalError(loc); 119 } 120 } 121 122 static void monitor_exit(jvmtiEnv* jvmti, JNIEnv* env, jrawMonitorID mon, const char* loc) { 123 jvmtiError err; 124 125 err = jvmti->RawMonitorExit(mon); 126 if (err != JVMTI_ERROR_NONE) { 127 ShowErrorMessage(jvmti, err, loc); 128 env->FatalError(loc); 129 } 130 } 131 132 static void monitor_wait(jvmtiEnv* jvmti, JNIEnv* env, jrawMonitorID mon, const char* loc) { 133 jvmtiError err; 134 135 err = jvmti->RawMonitorWait(mon, 0); 136 if (err != JVMTI_ERROR_NONE) { 137 ShowErrorMessage(jvmti, err, loc); 138 env->FatalError(loc); 139 } 140 } 141 142 static void monitor_notify(jvmtiEnv* jvmti, JNIEnv* env, jrawMonitorID mon, const char* loc) { 143 jvmtiError err; 144 145 err = jvmti->RawMonitorNotify(mon); 146 if (err != JVMTI_ERROR_NONE) { 147 ShowErrorMessage(jvmti, err, loc); 148 env->FatalError(loc); 149 } 150 } 151 152 static void monitor_destroy(jvmtiEnv* jvmti, JNIEnv* env, jrawMonitorID mon, const char* loc) { 153 jvmtiError err; 154 155 err = jvmti->DestroyRawMonitor(mon); 156 if (err != JVMTI_ERROR_NONE) { 157 ShowErrorMessage(jvmti, err, loc); 158 env->FatalError(loc); 159 } ... 160 } 196 while (target_thread == NULL) { 197 monitor_wait(jvmti, env, glws_monitor, AT_LINE); 198 } ... 220 while (test_state != TargetInNative) { 221 if (test_state == ShutDown) { 222 test_state = Terminated; 223 monitor_notify(jvmti, env, glws_monitor, AT_LINE); 224 monitor_exit(jvmti, env, glws_monitor, AT_LINE); 225 return; 226 } 227 monitor_wait(jvmti, env, glws_monitor, AT_LINE); 228 } ... 263 // Called by target thread after building a large stack. 264 // By calling this native method, the thread's stack becomes walkable. 265 // It notifies the agent to do the GetLocalObject() call and then races 266 // it to make its stack not walkable by returning from the native call. 267 JNIEXPORT void JNICALL 268 Java_GetLocalWithoutSuspendTest_notifyAgentToGetLocal(JNIEnv *env, jclass cls, jint depth, jlong waitCycles) { 269 jvmtiEnv* jvmti = jvmti_global; 270 271 monitor_enter(jvmti, env, glws_monitor, AT_LINE); 272 273 // Set depth_for_get_local and notify agent that the target thread is ready for the GetLocalObject() call 274 depth_for_get_local = depth; 275 test_state = TargetInNative; 276 277 monitor_notify(jvmti, env, glws_monitor, AT_LINE); 278 279 // Wait for agent thread to read depth_for_get_local and do the GetLocalObject() call 280 while (test_state != AgentInGetLocal) { 281 monitor_wait(jvmti, env, glws_monitor, AT_LINE); 282 } 283 284 // Reset state to Initial 285 test_state = Initial; 286 287 monitor_exit(jvmti, env, glws_monitor, AT_LINE); 288 289 // Wait a little until agent thread is in unsafe stack walk. 290 // This needs to be a spin wait or sleep because we cannot get a notification 291 // from there. 292 while (--waitCycles > 0) { 293 dummy_counter++; 294 } 295 } ... 299 JNIEXPORT void JNICALL 300 Java_GetLocalWithoutSuspendTest_shutDown(JNIEnv *env, jclass cls) { 301 jvmtiEnv* jvmti = jvmti_global; 302 303 monitor_enter(jvmti, env, glws_monitor, AT_LINE); 304 305 // Notify agent thread to shut down 306 test_state = ShutDown; 307 monitor_notify(jvmti, env, glws_monitor, AT_LINE); 308 309 // Wait for agent to terminate 310 while (test_state != Terminated) { 311 monitor_wait(jvmti, env, glws_monitor, AT_LINE); 312 } 313 314 monitor_exit(jvmti, env, glws_monitor, AT_LINE); 315 316 // Destroy glws_monitor 317 monitor_destroy(jvmti, env, glws_monitor, AT_LINE); 318 } Thanks, Serguei On 8/14/20 07:06, Reingruber, Richard wrote: Hi Serguei, thanks for the feedback. I have implemented your suggestions and created a new webrev: Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4/ Delta: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.4.inc/ Please find my replies to your comments below. Best regards, Richard. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.3.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java.frames.html 33 * the stack walk. The target thread's stack is walkable while in native. After sending the notification it ... 54 * @param depth Depth of target frame for GetLocalObject() call. Should be large value to prolong the unsafe stack walk. 55 * @param waitTimeInNativeAfterNotify Time to wait after notify with walkable stack before returning an becoming unsafe again. ... 71 * Wait time in native, i.e. with walkable stack, after notifying agent thread to do GetLocalObject() call. ... 89 msg((now -start) + " ms Iteration : " + iterations + " waitTimeInNativeAfterNotify : " + waitTimeInNativeAfterNotify); Could you, please, re-balance the lines above to make them shorter? Ok, done. 90 int newTargetDepth = recursiveMethod(0, targetDepth); 91 if (newTargetDepth < targetDepth) { 92 msg("StackOverflowError during test."); 93 msg("Old target depth: " + targetDepth); 94 msg("Retry with new target depth: " + newTargetDepth); 95 targetDepth = newTargetDepth; 96 } A comment is needed to explain why a StackOverflowError is not desired. At least, it is not obvious initially. 73 public int waitTimeInNativeAfterNotify; This name is unreasonably long which makes the code less readable. I'd suggest to reduce it to waitTime. Ok, done. 103 notifyAgentToGetLocalAndWaitShortly(-1, 1); ... 119 notifyAgentToGetLocalAndWaitShortly(depth - 100, waitTimeInNativeAfterNotify); It is better to provide a short comment before each call explaining what it is doing. For instance, it is not clear why the call at the line 103 is needed. Why do we need to notify the agent to GetLocal for the second time? The test is repeated TEST_ITERATIONS times. In each iteration the agent calls GetLocal racing the target thread returning from the native call. The last call in line 103 ist the shutdown signal. Can it be refactored into a separate native method? I've made the shutdown process more explicit with the new native method shutDown() which sets thest_state to ShutDown. Then the the function name can be reduced to 'notifyAgentToGetLocal'. This long name does not give enough context anyway. Ok, done. 85 long iterations = 0; 87 do { ... 97 iterations++; ... 102 } while (iterations < TEST_ITERATIONS); Why a more explicit 'for' or 'while' loop is not used here? : for (long iter = 0; iter < TEST_ITERATIONS; iter++) { I have converted the loop into a for loop. http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.3.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp.frames.html The indent in this file varies. It is better to keep it the same: 4 or 2. Yes, I noticed this. I have not corrected it yet, because I didn't want to pullute the incremental webrev with that change. Would you like me to fix the indentation now to 2 spaces or do it as a last step? 60 AgentCallingGetLocalObject // The target thread waits for the agent to call I'd suggest to rename the constant to 'AgentInGetLocal'. Ok, done. 150 GetLocalWithoutSuspendTestThreadLoop(jvmtiEnv * jvmti, JNIEnv* env, void * arg) { It is better rename the function to TestThreadLoop. Would AgentThreadLoop be ok too? You can add a comment before to explain some basic about what it is doing. Ok, done. 167 printf("*** AGENT: GetLocalWithoutSuspendTestThreadLoop thread started. Polling thread '%s' for local variables\n", It is better to get rid of leading stars in all messages. Ok, done. 176 // the native method Java_GetLocalWithoutSuspendTest_notifyAgentToGetLocalAndWaitShortly The part 'Java_GetLocalWithoutSuspendTest_' can be removed from the function name. Ok, done. --- From: mailto:serguei.spit...@oracle.com mailto:serguei.spit...@oracle.com Sent: Freitag, 14. August 2020 10:11 To: Reingruber, Richard mailto:richard.reingru...@sap.com; David Holmes mailto:david.hol...@oracle.com; mailto:serviceability-dev@openjdk.java.net Subject: Re: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() Hi Richard, http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.3.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java.frames.html 33 * the stack walk. The target thread's stack is walkable while in native. After sending the notification it ... 54 * @param depth Depth of target frame for GetLocalObject() call. Should be large value to prolong the unsafe stack walk. 55 * @param waitTimeInNativeAfterNotify Time to wait after notify with walkable stack before returning an becoming unsafe again. ... 71 * Wait time in native, i.e. with walkable stack, after notifying agent thread to do GetLocalObject() call. ... 89 msg((now -start) + " ms Iteration : " + iterations + " waitTimeInNativeAfterNotify : " + waitTimeInNativeAfterNotify); Could you, please, re-balance the lines above to make them shorter? 90 int newTargetDepth = recursiveMethod(0, targetDepth); 91 if (newTargetDepth < targetDepth) { 92 msg("StackOverflowError during test."); 93 msg("Old target depth: " + targetDepth); 94 msg("Retry with new target depth: " + newTargetDepth); 95 targetDepth = newTargetDepth; 96 } A comment is needed to explain why a StackOverflowError is not desired. At least, it is not obvious initially. 73 public int waitTimeInNativeAfterNotify; This name is unreasonably long which makes the code less readable. I'd suggest to reduce it to waitTime. 103 notifyAgentToGetLocalAndWaitShortly(-1, 1); ... 119 notifyAgentToGetLocalAndWaitShortly(depth - 100, waitTimeInNativeAfterNotify); It is better to provide a short comment before each call explaining what it is doing. For instance, it is not clear why the call at the line 103 is needed. Why do we need to notify the agent to GetLocal for the second time? Can it be refactored into a separate native method? Then the the function name can be reduced to 'notifyAgentToGetLocal'. This long name does not give enough context anyway. 85 long iterations = 0; 87 do { ... 97 iterations++; ... 102 } while (iterations < TEST_ITERATIONS); Why a more explicit 'for' or 'while' loop is not used here? : for (long iter = 0; iter < TEST_ITERATIONS; iter++) { http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.3.inc/test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp.frames.html The indent in this file varies. It is better to keep it the same: 4 or 2. 60 AgentCallingGetLocalObject // The target thread waits for the agent to call I'd suggest to rename the constant to 'AgentInGetLocal'. 150 GetLocalWithoutSuspendTestThreadLoop(jvmtiEnv * jvmti, JNIEnv* env, void * arg) { It is better rename the function to TestThreadLoop. You can add a comment before to explain some basic about what it is doing. 167 printf("*** AGENT: GetLocalWithoutSuspendTestThreadLoop thread started. Polling thread '%s' for local variables\n", It is better to get rid of leading stars in all messages. 176 // the native method Java_GetLocalWithoutSuspendTest_notifyAgentToGetLocalAndWaitShortly The part 'Java_GetLocalWithoutSuspendTest_' can be removed from the function name. I'm still reviewing the test native agent code. Thanks, Serguei On 8/11/20 03:02, Reingruber, Richard wrote: Hi David and Serguei, On 11/08/2020 3:21 am, mailto:serguei.spit...@oracle.com wrote: Hi Richard and David, The implementation looks good to me. But I do not understand what the test is doing with all this counters and recursions. For instance, these fragments: 86 recursions = 0; 87 try { 88 recursiveMethod(1<<20); 89 } catch (StackOverflowError e) { 90 msg("Caught StackOverflowError as expected"); 91 } 92 int target_depth = recursions-100; // spaces are missed around the '-' sigh It is not obvious that the 'recursion' is updated in the recursiveMethod. I would suggestto make it more explicit: recursiveMethod(M); int target_depth = M - 100; Then the variable 'recursions' can be removed or become local. The recursiveMethod takes in the maximum recursions to try and updates the recursions variable to record how many recursions were possible - so: target_depth = <actual recursions> - 100; Possibly recursiveMethod could return the actual recursions instead of using the global variables? I've eliminated the static 'recursions' variable. recursiveMethod() now returns the depth at which the recursion was ended. I hesitated doing this, because I had to handle the StackOverflowError with all those frames still on stack. But the handler is empty, so it should not cause problems. This is the new webrev (as posted previously): Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.3/ Delta: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.3.inc/ Thanks, Richard. -----Original Message----- From: David Holmes mailto:david.hol...@oracle.com Sent: Dienstag, 11. August 2020 04:00 To: mailto:serguei.spit...@oracle.com; Reingruber, Richard mailto:richard.reingru...@sap.com; mailto:serviceability-dev@openjdk.java.net Subject: Re: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() Hi Serguei, On 11/08/2020 3:21 am, mailto:serguei.spit...@oracle.com wrote: Hi Richard and David, The implementation looks good to me. But I do not understand what the test is doing with all this counters and recursions. For instance, these fragments: 86 recursions = 0; 87 try { 88 recursiveMethod(1<<20); 89 } catch (StackOverflowError e) { 90 msg("Caught StackOverflowError as expected"); 91 } 92 int target_depth = recursions-100; // spaces are missed around the '-' sigh It is not obvious that the 'recursion' is updated in the recursiveMethod. I would suggestto make it more explicit: recursiveMethod(M); int target_depth = M - 100; Then the variable 'recursions' can be removed or become local. The recursiveMethod takes in the maximum recursions to try and updates the recursions variable to record how many recursions were possible - so: target_depth = <actual recursions> - 100; Possibly recursiveMethod could return the actual recursions instead of using the global variables? David ----- This method will be: 47 private static final int M = 1 << 20; ... 121 public long recursiveMethod(int depth) { 123 if (depth == 0) { 124 notifyAgentToGetLocalAndWaitShortly(M - 100, waitTimeInNativeAfterNotify); 126 } else { 127 recursiveMethod(--depth); 128 } 129 } At least, he test is missing the comments explaining all these. Thanks, Serguei On 8/9/20 22:35, David Holmes wrote: Hi Richard, On 31/07/2020 5:28 pm, Reingruber, Richard wrote: Hi, I rebase the fix after JDK-8250042. New webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.2/ The general fix for this seems good. A minor nit: 588 if (!is_assignable(signature, ob_k, Thread::current())) { You know that the current thread is the VMThread so can use VMThread::vm_thread(). Similarly for this existing code: 694 Thread* current_thread = Thread::current(); --- Looking at the test code ... I'm less clear on exactly what is happening and the use of spin-waits raises some red-flags for me in terms of test reliability on different platforms. The "while (--waitCycles > 0)" loop in particular offers no certainty that the agent thread is executing anything in particular. And the use of the spin_count as a guide to future waiting time seems somewhat arbitrary. In all seriousness I got a headache trying to work out how the test was expecting to operate. Some parts could be simplified using raw monitors, I think. But there's no sure way to know the agent thread is in the midst of the stackwalk when the target thread wants to leave the native code. So I understand what you are trying to achieve here, I'm just not sure how reliably it will actually achieve it. test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/libGetLocalWithoutSuspendTest.cpp 32 static volatile jlong spinn_count = 0; Using a 64-bit counter seems like it will be a problem on 32-bit systems. Should be spin_count not spinn_count. 36 // Agent thread waits for value != 0, then performas the JVMTI call to get local variable. typo: performas Thanks, David ----- Thanks, Richard. -----Original Message----- From: serviceability-dev mailto:serviceability-dev-r...@openjdk.java.net On Behalf Of Reingruber, Richard Sent: Montag, 27. Juli 2020 09:45 To: mailto:serguei.spit...@oracle.com; mailto:serviceability-dev@openjdk.java.net Subject: [CAUTION] RE: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() Hi Serguei, > I tested it on Linux and Windows but not yet on MacOS. The test succeeded now on all platforms. Thanks, Richard. -----Original Message----- From: Reingruber, Richard Sent: Freitag, 24. Juli 2020 15:04 To: mailto:serguei.spit...@oracle.com; mailto:serviceability-dev@openjdk.java.net Subject: RE: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() Hi Serguei, The fix itself looks good to me. thanks for looking at the fix. I still need another look at new test. Could you, please, convert the agent of new test to C++? It will make it a little bit simpler. Sure, here is the new webrev.1 with a C++ version of the test agent: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.1/ I tested it on Linux and Windows but not yet on MacOS. Thanks, Richard. -----Original Message----- From: mailto:serguei.spit...@oracle.com mailto:serguei.spit...@oracle.com Sent: Freitag, 24. Juli 2020 00:00 To: Reingruber, Richard mailto:richard.reingru...@sap.com; mailto:serviceability-dev@openjdk.java.net Subject: Re: RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue() Hi Richard, Thank you for filing the CR and taking care about it! The fix itself looks good to me. I still need another look at new test. Could you, please, convert the agent of new test to C++? It will make it a little bit simpler. Thanks, Serguei On 7/20/20 01:15, Reingruber, Richard wrote: Hi, please help review this fix for VM_GetOrSetLocal. It moves the unsafe stackwalk from the vm operation prologue before the safepoint into the doit() method executed at the safepoint. Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.0/index.html Bug: https://bugs.openjdk.java.net/browse/JDK-8249293 According to the JVMTI spec on local variable access it is not required to suspend the target thread T [1]. The operation will simply fail with JVMTI_ERROR_NO_MORE_FRAMES if T is executing bytecodes. It will succeed though if T is blocked because of synchronization or executing some native code. The issue is that in the latter case the stack walk in VM_GetOrSetLocal::doit_prologue() to prepare the access to the local variable is unsafe, because it is done before the safepoint and it races with T returning to execute bytecodes making its stack not walkable. The included test shows that this can crash the VM if T wins the race. Manual testing: - new test test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java - test/hotspot/jtreg/vmTestbase/nsk/jvmti - test/hotspot/jtreg/serviceability/jvmti Nightly regression tests @SAP: JCK and JTREG, also in Xcomp mode, SPECjvm2008, SPECjbb2015, Renaissance Suite, SAP specific tests with fastdebug and release builds on all platforms Thanks, Richard. [1] https://docs.oracle.com/en/java/javase/14/docs/specs/jvmti.html#local
