Towards Automating the Redesign of the Synchronization Skeletons of UPC Programs

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Towards Automating the Redesign of the
Synchronization Skeletons of UPC Programs
Presenter Amin Alipour Adviser Ali Ebnenasir
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Motivations

• Help developers in
• automated behavioral debugging, and
• design of synchronization mechanisms of
  multithreaded programs, i.e., synchronization
  skeleton

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Example A UPC Program
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Problem

• Deadlock
• Need deadlock-freedom

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Outline

• Problem Statement
• Proposed Solution
• Case Study

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Problem Statement

• Input
• A UPC program P
• A desired mutual exclusion (e.g., lack of data
  race) or progress property (e.g.,
  deadlock-freedom), denoted L
• P does not meet L
• Output
• A revised version of P, denoted P_r
• Constraints
• P_r meets L
• P_r does not violate other properties of P

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Proposed Solution
UPC Program
Revised UPC Program
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Synchronization Skeleton of the UPC Program

• Synch. Skeleton of thread i
• // Generate an array element index 0/1
• TS_i
• upc_lock(lki)
• upc_lock(lks)
• CS_i
• // Swap
• ...
• // Dont forget to unlock
• upc_unlock(lki)
• upc_unlock(lks)

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Transforming Synchronization Skeleton to Guarded
Commands
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Guarded Commands

• State snapshot valuation to program variables
• Guarded command
• ltgrd_1gt ? stmt_1
• ltgrd_2gt ? stmt_2
• ltgrd_ngt ? stmt_n
• Example
• (x gt y) ? x x - y
• (y gt x) ? y y - x
• A set of transitions (s0, s1)?
• grd is true in s0, and
• s1 is reach by atomic execution of stmt

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Canonical UPC Programs

• For simplicity, we consider a canonical version
  of the program with
• 2 threads
• An array with size 2

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Case Study-State

TSi Trying Section Thread i CSi Critical
Section Thread i LKi Lock of Thread
i GLKi Lock for Intra-thread Serialization
Thread i Si random value selected by Thread
i SSi Whether Thread i has selected a random
value
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Translation of Synchronization Skeleton to
Guarded Commands
Initial state values TS_i true CS_i
false sSet_i false Ai1 (sSet_i false)
-gt s_i 01 sSet_i true Ai2
(sSet_i true) /\ (TS_i true) /\ (lki
true) /\ (gotLock_i false) -gt lki
false gotLock_i true Ai3
(sSet_i true) /\ (TS_i true) /\ (lks_i
true) /\ gotLock_i true -gt lks_i
false gotLock_i false
TS_i false CS_i
true Ai4 (sSet_i true) /\ (CS_i true) /\
(lki false) /\ (lks_i false) -gt
lki true lks_i true CS_i
falseTS_i true sSet_i false
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Generating Reachability Graph for the program
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Reachability Graph Generation

• Start with initial states
• Check applicable commands
• Generate new reachable states
• run Steps 1 and 2 until no new state is generated

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Finite Model
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Finite Model
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Finite Model
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Finite Model
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Automatic Revision
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Revised Model
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Revised Thread 1
Thread 1 A11 (!SS1 ? !S2) ? (!SS2 !S1)
-gtS11 SS1 true A12 (TS1 ?!CS1 ? LK0
?!GLK1 ?(S1) ? SS1)-gtLK0 falseGLK1
true A13 (TS1 ?!CS1 ?!LK0 ? GLK1 ?(S1) ?
SS1)-gtLK1 false GLK1falseTS1 falseCS1
true A14 (!TS1 ? CS1 ?!LK0 ?!GLK1 ?(S1) ? SS1
? SS2)-gtLK0 true LK1 true CS1
falseTS1 trueSS1 false
Avoid s Deadlock
Ai1 (sSet_i false) -gt s_i 01 sSet_i
true Ai2 (sSet_i true) ? (TS_i true) ?
(lki true) ? (gotLock_i false) -gt
lki false gotLock_i true Ai3 (sSet_i
true) ? (TS_i true) ? (lks_i true) ?
gotLock_i true -gt
lks_i false gotLock_i falseTS_i
falseCS_i true Ai4 (sSet_i true) ? (CS_i
true) ? (lki false) ? (lks_i false) -gt
lki true lks_i true CS_i
falseTS_i true sSet_i false
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Avoids Deadlock
Thread 2? A21 !SS2 -gtS20 SS2 true A22
TS2 ? !CS2 ? LK1 ? !GLK2 ? !(S2) ? S1 ? SS2
? !SS1 -gtLK1 falseGLK2 true A23 TS2 ?
!CS2 ? !LK1 ? GLK2 ? !(S2) ? SS2 -gtLK0
false GLK2falseTS2 falseCS2 true A24
!TS2 ? CS2 ? !LK1 ? !GLK2 ? !(S2) ? SS2
-gtLK0 true LK1 true CS2 falseTS2
trueSS2 false
Imposes sequential execution
Ai1 (sSet_i false) -gt s_i 01
sSet_i true Ai2 (sSet_i true) ?
(TS_i true) ? (lki true) ? (gotLock_i
false) -gt lki false
gotLock_i true Ai3 (sSet_i true) ? (TS_i
true) ? (lks_i true) ? gotLock_i true -gt
lks_i false gotLock_i falseTS_i
false CS_i true Ai4 (sSet_i
true) ? (CS_i true) ? (lki false) ?
(lks_i false) -gt lki true lks_i
true CS_i falseTS_i true sSet_i false
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Questions