Link to TSL instruction

Question

I want to use the "TSL" instruction in the assembly, but there are no links to understand it. In some articles, this instruction is introduced for the problem of mutual exclusion, but it does not have a complete or comprehensive example to fully understand.

Answer 1

TSL (Test and Set Lock) is an operation that often occurs when solving mutual exclusion problems in general, but this does not mean that such an instruction really exists for any architecture that you use; or, even if it exists, that it is called TSL.

In x86, for example, you can use the XCHG instruction to execute TSL.

Answer 2

XCNG is not conditional, it just swaps case with memory location. You want CMPXCHG with the prefix LOCK. The latter will make it atomic on multi-core machines.

Alternatively, you can implement atomic comparison and typing with LOCK XADD, but it will take a loop.

Link: http://en.wikipedia.org/wiki/Compare-and-swap

Answer 3

I think @Jester and @Seva Alekseyev have already done an excellent job answering this question, but here is a simple C implementation with built-in build using pthreads on an x86 Ubuntu machine.

The following example has two long streams. Both threads have a critical and non-critical section, critical_region () and non-critical region, respectively. They both must call enter_region () to get the lock. As soon as a thread receives a lock, it can start executing its critical section. The other thread blocks until the thread with the lock calls leave_region () .

 #include <stdio.h> #include <unistd.h> #include <pthread.h> #include <stdlib.h> void enter_region() { asm( ".data\n\t" "lock:\n\t" ".byte 0\n\t" ".text\n\t" "_enter_region:\n\t" "movb $1, %al\n\t" /* move 1 to AL */ "xchgb (lock),%al\n\t" "cmp $0, %al\n\t" "jne _enter_region\n\t" ); } void leave_region() { asm("movb $0, (lock)"); } void critical_region() { } void noncritical_region() { } static void* f1(void* p) { while(1) { puts("wait for f2"); enter_region(); printf("f1 can start its critical section\n"); critical_region(); leave_region(); noncritical_region(); } return NULL; } static void* f2(void* p) { while(1) { puts("wait for f1"); enter_region(); printf("f2 can start its critical section\n"); critical_region(); leave_region(); /* if you call sleep, you can see that the non-critical section of this thread won't * block the other thread from running its critical section as many times as it wants */ // sleep(1); noncritical_region(); } return NULL; } int main() { int rc; pthread_t t1, t2; rc = pthread_create(&t1, NULL, f1, NULL); if(rc != 0) { fprintf(stderr, "pthread f1 failed\n"); return EXIT_FAILURE; } rc = pthread_create(&t2, NULL, f2, NULL); if(rc != 0) { fprintf(stderr, "pthread f2 failed\n"); return EXIT_FAILURE; } pthread_join(t1, NULL); pthread_join(t2, NULL); puts("All threads finished."); return 0; }