Lecture Notes in Computer Science 3472

18 Run-Time Verification 533
the empty set, therefore no locks protects v. A warning is issued since accesses
to variable v are sometimes protected by the lock mu1 and sometimes by the
lock mu2, thus no locks protects all accesses to v for the whole computation.
⏐
Program Locks held C (v)
⏐
{} {mu1,mu2}
⏐
lock(mu1)
⏐
{mu1} {mu1,mu2}
⏐
v := v+1
⏐
{mu1} {mu1}
⏐
Time ⏐ unlock(mu1)
⏐
{} {mu1}
⏐ lock(mu2)
⏐
{mu2} {mu1}
⏐ v := v+1
⏐
{m2} {}
⏐ unlock(mu2)
�
{} {}
Fig. 18.3. Example of the execution progress for the Eraser algorithm in the case of a
variable v used by a six statements program.
When a race is reported, Eraser indicates both the file and line number at
which the instruction violating the locking discipline is located. The report also
includes the thread ID, memory address, type of memory access, and important
register values such as the program counter and stack pointer.
The Eraser locking discipline is too strong. There are three very common
programming practices that violate the discipline, yet are free from any data
races: initialization, read-shared data and read-write locks. These cases have been
addressed by Savage et al. who extended the Lockset algorithm to accommodate
initialization, read-shared data and read-write locks [SBN + 97].
The Eraser algorithm was implemented for Digital Unix and it has been
used to detect data races in several programs. In particular, it tackled some
large multi-threaded servers written by experienced researchers at Digital Equipment
Corporation’s System Research Center: the HTTP server and indexing
engine from AltaVista, the Vesta cache server, and the Petal distributed disk
system. Eraser found undesirable race conditions in three of the four server programs
[SBN + 97]. A version of the Eraser algorithm which addresses also Java
programs has been implemented in PathExplorer [HR01b]. Finally, Compaq provides
a run-time debugging and analysis tool for multi-threaded applications
called Visual Threads which finds data races by the Eraser algorithm [Har00].
18.4.2 Deadlock Detection
The problem of deadlock is common in parallel programs. Deadlock can occur
whenever shared resources are required to accomplish a task. For instance, if