S. Narayanasamy, Z. Wang, J. Tigani, A. Edwards, B. Calder UCSD and Microsoft PLDI 2007

 Data Races hard to debug ◦ Difficult to detect ◦ Even more difficult to reproduce  Data Race Detectors help in detection ◦ LockSet, Happens-Before and Atomicity Violation  But they tend to overdo it ◦ Up to 90% false alarms  Especially with LockSet We need a tool that detects and reliably classifies all harmful Data Races 2

 Offline Dynamic Happens-Before Data Race Detection ◦ Step 1: Trace Capturing ◦ Step 2: Offline Happens-Before Analysis ◦ Step 3: Replay Critical Segments ◦ Step 4: Auto Classify harmful vs. benign races 3

 iDNA captures the execution of an application  Simply records initial state, ◦ Registers and PC  load values, ◦ Only those needed absolutely ◦ 1 st load after a store, DMA etc…  and a global clock (sequensers) ◦ Inserted in the thread’s replay log for  Synchronization events  System calls 4

5  Good old Happens- Before ◦ Two conflicting accesses  At least one write  Not ordered  Detects only the data races that happened

 When a data race is detected replay the affected segments twice ◦ 1 st with the actual order  Given by the load values ◦ 2 nd reverse the racing accesses  Store the replay result ◦ No-State-Change: If all live-outs are the same ◦ State-Change: If at least 1 live-out changed ◦ Replay Failure: If disaster encountered  Load null or unencountered address  Branch someplace else 6

7 Replay Failure Potentially Harmful Data Race

 Repeat step 3 for each instance of a data race  Potentially Benign Data Race ◦ every replay results to No-State-Change  Potentially Harmful Data Race ◦ ≥1 replay results in State-Change or Replay Failure  State-Change shows that something would be different if things took the other path  Replay Failure indicates that a program changed that much, so we cannot simulate the other state ◦ Concrete proof that something definitely changed  Easier for the programmers to accept it 8

 18 different executions of various services in Windows Vista and Internet Explorer  Happens-Before returns 16,642 data races ◦ 68 unique  Trace capture ◦ 0.8 bits per instruction  96 MB per 1,000,000,000 instructions  Only 1 st loads and synchronizers captured ◦ 0.3 if compressed with zip 9

 Results for Internet Explorer ◦ P4 Xeon 2.2 GHz, 1 GB of RAM  Start adding… ◦ 6x for capturing ◦ 10x for replaying (unnecessary) ◦ 45x offline Happens-Before Data Race Detection ◦ 280x replay analysis  2,196 dynamic data races 10

Potentially BenignPotentially Harmful To- tal Real BenignReal HarmfulReal BenignReal Harmful No-State- Change 320 State Change Replay Failure Total Impossible State Automatically Classified Manually Classified All harmful races identified correctly 0 false negatives All harmful races identified correctly 0 false negatives Half benign races identified correctly. Half still persist

 32 Real Benign races classified as such ◦ Every instance must return No-State-Changed ◦ The more instances, the more confidence in the classification 12

 7 Real bugs, correctly identified  At least 1 State- Change or Replay Failure required 13 Dangerous Zone

 29 Benign races incorrectly classified as harmful ◦ Approximate Computation (23/29)  Statistics etc ◦ Replayer Limitation (6/29)  At least 1 instance caused replay failure  The final outcome is the same 14

 User Constructed ◦ Garbage collector does not use locks  Double Checks ◦ If (a) {lock(…); if(a) {…}}  Both Values Valid ◦ Use cache? High Perf?  Redundant Writes ◦ Rewrite the same value  Disjoint bit manipulation ◦ Modify different bits in same variable # Races User Constructed Synchronization 8 Double Checks3 Both Values Valid5 Redundant Writes13 Disjoint bit manipulation 9 Approx. Computation false positives that were not caused by replay failure

 Interesting approach to identify benign races  It would be interesting to apply it to LockSet ◦ LockSet has far more false positives ◦ But it can detect bugs that did not happen in production runs  A grand total overhead is missing 16

17 Thank You!!!