Shared Variables in Threaded C Programs 15 213 The course that gives CMU its Zip Question Which variables in a threaded C program are shared variables Programming with Threads Dec 5 2002 Shared variables n The need for synchronization n Synchronizing with semaphores n Thread safety and reentrancy n Races and deadlocks The answer is not as simple as global variables are shared and stack variables are private Requires answers to the following questions Topics n n n What is the memory model for threads n How are variables mapped to memory instances n How many threads reference each of these instances 15 213 F 02 2 class29 ppt Example of Threads Accessing Another Thread s Stack Threads Memory Model Conceptual model n n char ptr Each thread runs in the context of a process Each thread has its own separate thread context l Thread ID stack stack pointer program counter condition codes and general purpose registers n All threads share the remaining process context l Code data heap and shared library segments of the process virtual address space l Open files and installed handlers Operationally this model is not strictly enforced n n 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 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 Shared Variable Analysis 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 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 15 213 F 02 badcnt c An Improperly Synchronized Threaded Program unsigned int cnt 0 shared NULL NULL NULL NULL Pthread join tid1 NULL Pthread join tid2 NULL 7 if cnt unsigned NITERS 2 printf BOOM cnt d n cnt else printf OK cnt d n cnt thread routine void count void arg int i for i 0 i NITERS i cnt return NULL 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 Local static var 1 instance svar data 5 int main pthread t tid1 tid2 Pthread create tid1 count Pthread create tid2 count Which variables are shared n ptr svar and msgs are shared n i and myid are NOT shared 15 213 F 02 6 Assembly Code for Counter Loop C code for counter loop L9 Head Hi linux badcnt BOOM cnt 198841183 linux badcnt BOOM cnt 198261801 Load cnt Li Update cnt Ui Store cnt Si linux badcnt BOOM cnt 198269672 Tail Ti cnt should be equal to 200 000 000 What went wrong 15 213 F 02 Corresponding asm code gcc O0 fforce mem for i 0 i NITERS i cnt L12 L11 L10 8 movl 4 ebp eax cmpl 99999999 eax jle L12 jmp L10 movl cnt eax leal 1 eax edx movl edx cnt Load Update Store movl 4 ebp eax leal 1 eax edx movl edx 4 ebp jmp L9 15 213 F 02 Concurrent Execution Concurrent Execution cont Key idea In general any sequentially consistent interleaving is possible but some are incorrect Incorrect ordering two threads increment the counter but the result is 1 instead of 2 n Ii denotes that thread n eaxi is the contents i executes instruction I 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 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 15 213 F 02 10 Concurrent Execution cont Progress Graphs How about this ordering Thread 2 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 A progress graph depicts the discrete execution state space of concurrent threads cnt 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 We can clarify our understanding of concurrent execution with the help of the progress graph 11 Oops OK 15 213 F 02 9 i thread 15 213 F 02 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 Critical Sections and Unsafe Regions Thread 2 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 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 Unsafe region U2 L2 Sets of states where such interleaving occurs form unsafe regions L2 H2 H2 H1 L1 U1 S1 Thread 1 T1 H1 L1 U1 S1 T1 Thread 1 critical section wrt cnt 15 213 F 02 13 15 213 F 02 14 Semaphores Safe and Unsafe Trajectories Thread 2 Question How can we guarantee a safe trajectory n 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 Classic solution solution Dijkstra s P and V operations on semaphores n Claim A trajectory is correct wrt cnt iff it is safe U2 We must synchronize the threads so that they never enter an unsafe state semaphore non negative integer synchronization variable l P s while s 0 wait s Dutch for …
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