15 213 The course that gives CMU its Zip Programming with Threads Dec 5 2002 Topics class29 ppt Shared variables The need for synchronization Synchronizing with semaphores Thread safety and reentrancy Races and deadlocks Shared Variables in Threaded C Programs Question Which variables in a threaded C program are shared variables The answer is not as simple as global variables are shared and stack variables are private Requires answers to the following questions 2 What is the memory model for threads How are variables mapped to memory instances How many threads reference each of these instances 15 213 F 02 Threads Memory Model Conceptual model Each thread runs in the context of a process Each thread has its own separate thread context Thread ID stack stack pointer program counter condition codes and general purpose registers All threads share the remaining process context Code data heap and shared library segments of the process virtual address space Open files and installed handlers Operationally this model is not strictly enforced While register values are truly separate and protected Any thread can read and write the stack of any other thread Mismatch between the conceptual and operation model is a source of confusion and errors 3 15 213 F 02 Example of Threads Accessing Another Thread s Stack char ptr global int main int i pthread t tid char msgs N Hello from foo Hello from bar ptr msgs for i 0 i 2 i Pthread create tid NULL thread void i Pthread exit NULL 4 thread routine void thread void vargp int myid int vargp static int svar 0 printf d s svar d n myid ptr myid svar Peer threads access main thread s stack indirectly through global ptr variable 15 213 F 02 Mapping Variables to Mem Instances Global var 1 instance ptr data Local automatic vars 1 instance i m msgs m char ptr global int main int i pthread t tid char msgs N Hello from foo Hello from bar ptr msgs for i 0 i 2 i Pthread create tid NULL thread void i Pthread exit NULL 5 Local automatic var 2 instances myid p0 peer thread 0 s stack myid p1 peer thread 1 s stack thread routine void thread void vargp int myid int vargp static int svar 0 printf d s svar d n myid ptr myid svar Local static var 1 instance svar data 15 213 F 02 Shared Variable Analysis Which variables are shared Variable instance Referenced by main thread Referenced by peer thread 0 Referenced by peer thread 1 ptr svar i m msgs m myid p0 myid p1 yes no yes yes no no yes yes no yes yes no yes yes no yes no yes Answer A variable x is shared iff multiple threads reference at least one instance of x Thus 6 ptr svar and msgs are shared i and myid are NOT shared 15 213 F 02 badcnt c An Improperly Synchronized Threaded Program unsigned int cnt 0 shared int main pthread t tid1 tid2 Pthread create tid1 count Pthread create tid2 count NULL NULL NULL NULL Pthread join tid1 NULL Pthread join tid2 NULL if cnt unsigned NITERS 2 printf BOOM cnt d n cnt else printf OK cnt d n cnt 7 thread routine void count void arg int i for i 0 i NITERS i cnt return NULL linux badcnt BOOM cnt 198841183 linux badcnt BOOM cnt 198261801 linux badcnt BOOM cnt 198269672 cnt should be equal to 200 000 000 What went wrong 15 213 F 02 Assembly Code for Counter Loop C code for counter loop Corresponding asm code gcc O0 fforce mem for i 0 i NITERS i cnt L9 movl 4 ebp eax cmpl 99999999 eax jle L12 jmp L10 Head Hi Load cnt Li Update cnt Ui Store cnt Si L12 movl cnt eax leal 1 eax edx movl edx cnt Load Update Store L11 movl 4 ebp eax leal 1 eax edx movl edx 4 ebp jmp L9 Tail Ti L10 8 15 213 F 02 Concurrent Execution Key idea In general any sequentially consistent interleaving is possible but some are incorrect 9 Ii denotes that thread i executes instruction I eaxi is the contents of eax in thread i s context i thread instri eax1 eax2 cnt 1 1 1 1 2 2 2 2 2 1 H1 L1 U1 S1 H2 L2 U2 S2 T2 T1 0 1 1 1 1 2 2 2 0 0 0 1 1 1 1 2 2 2 OK 15 213 F 02 Concurrent Execution cont Incorrect ordering two threads increment the counter but the result is 1 instead of 2 10 i thread instri eax1 eax2 cnt 1 1 1 2 2 1 1 2 2 2 H1 L1 U1 H2 L2 S1 T1 U2 S2 T2 0 1 1 1 0 1 1 1 0 0 0 0 0 1 1 1 1 1 Oops 15 213 F 02 Concurrent Execution cont How about this ordering i thread instri 1 1 2 2 2 2 1 1 1 2 H1 L1 H2 L2 U2 S2 U1 S1 T1 T2 eax1 eax2 cnt We can clarify our understanding of concurrent execution with the help of the progress graph 11 15 213 F 02 Progress Graphs A progress graph depicts the discrete execution state space of concurrent threads Thread 2 T2 L1 S2 Each axis corresponds to the sequential order of instructions in a thread S2 U2 Each point corresponds to a possible execution state Inst1 Inst2 L2 H2 H1 12 L1 U1 S1 T1 Thread 1 E g L1 S2 denotes state where thread 1 has completed L1 and thread 2 has completed S2 15 213 F 02 Trajectories in Progress Graphs Thread 2 A trajectory is a sequence of legal state transitions that describes one possible concurrent execution of the threads T2 S2 Example U2 H1 L1 U1 H2 L2 S1 T1 U2 S2 T2 L2 H2 H1 13 L1 U1 S1 T1 Thread 1 15 213 F 02 Critical Sections and Unsafe Regions Thread 2 L U and S form a critical section with respect to the shared variable cnt T2 Instructions in critical sections wrt to some shared variable should not be interleaved S2 critical section wrt cnt U2 Unsafe region Sets of states where such interleaving occurs form unsafe regions L2 H2 H1 L1 U1 S1 T1 Thread 1 critical section wrt cnt 14 15 213 F 02 Safe and Unsafe Trajectories Thread 2 T2 Safe trajectory S2 critical section wrt cnt Unsafe trajectory Unsafe region Def A trajectory is safe iff it doesn t touch any part of an unsafe region Claim A trajectory is correct wrt cnt iff it is safe U2 L2 H2 H1 L1 U1 S1 T1 Thread 1 critical section wrt cnt 15 15 213 F 02 Semaphores Question How can we guarantee a safe trajectory We must synchronize the threads so that they never enter an unsafe state Classic solution Dijkstra s P and V operations on semaphores semaphore non negative integer synchronization variable P s while s 0 wait s Dutch for Proberen test V s s Dutch for …
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