Eraser: A Dynamic Data Race Detector for Multi-Threaded Programs - Prof. Alan L. Sussman, Study notes of Computer Science

Download Eraser: A Dynamic Data Race Detector for Multi-Threaded Programs - Prof. Alan L. Sussman and more Study notes Computer Science in PDF only on Docsity!

Eraser

Eraser: A Dynamic Data Race Detector for Multi-Threaded Programs S. Savage, M. Burrows, G. Nelson, P. Sobalvarro, T. Anderson Serge Koren CMSC

Outline ➲ (^) Introduction ➲ (^) Related approaches ➲ (^) Eraser's approach ➲ (^) Implementation/Experience ➲ (^) Conclusion

Related Approaches ● Monitors – state and procedures together ● Happens-before ● Events ordered between threads based on synchronization objects they access ● More general, more costly ● Less thorough

Happens-before

Example ● (^) Program Locks_held C(v) {} {mu1, mu2} lock(mu1); v = v + 1; {mu1} {mu1} unlock(mu1); {} lock(mu2); v = v + 1; {mu2} unlock(mu2); {}, WARNING

Improving Lockset ● Initialization – initialize a shared variable without holding lock ● Read-shared data – written only at initialization and then read (no need for locks) ● Read-write locks – read by multiple threads, written by only one at a time

Read-Write Locks ● Lock m protects v if m held in write for writes and in read/write for read ● On each read of variable, v, by thread, t ● Set C(v) = C(v) ∩ locks_held(t) ● (^) If C(v) = {}, issue warning ● On each write of variable, v, by thread, t ● Set C(v) = C(v) ∩ write_ locks_held(t) ● If C(v) = {}, issue warning

Implementation ● Instrument program ● Indexed table of lock sets (hashed and sorted) ● Shadow word ● Exists for every 32-bit data segment ● (^) State condition ● (^) Lockset index ● 10-30 times slower than without

Experience ● Tested on different code bases ● (^) Found true data races, found false alarms ● Effectiveness/Sensitivity ● Found data races that had existed before ● Works the same on two or ten threads ● Multiple locks ● Many read locks, write takes all read locks ● Deadlock detection using ordering of locks

Conclusion ● Little data on sensitivity ● Only works for locks (pthreads) ● Annotations needed ● Slow/doubled memory usage ● Multiple lock fix incorrect/unclear ● http://valgrind.org (Helgrind: a data-race detector)