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The issue is you don't actually know that the thread calling gate_Enter is the same one that originally entered the gate. If another thread calls exampleUse function it's using the same pass. You could get around this by making use of thread local storage for your pass/checkin variable. Or you could create exampleUsePass on the stack, then pass it in to recursive calls instead of using global vars.

The issue is you don't actually know that the thread calling gate_Enter is the same one that originally entered the gate. If another thread calls exampleUse function it's using the same pass. You could get around this by making use of thread local storage for your pass/checkin variable. Or you could create exampleUsePass on the stack, then pass it in to recursive calls instead of using global vars.

A slightly more robust version of your code, which will abort if there is an unexpected exit from gate when you don't own the gate is shown below (along with a simple test harness).

#include <stdio.h>
#include <unistd.h>
#include <unistd.h>
#include <string.h>
#include <pthread.h>
#define gate_Gate volatile int
#define gate_Pass volatile int
/*extern*/ inline void
gate_Enter
(gate_Gate *gate, gate_Pass *pass)
{
 asm volatile (
 "jmp gate_Enter_check\n" // Skip the line.
 "gate_Enter_wait:\n" // Wait in line.
 "pause\n" // Wait a long time.
 "gate_Enter_check:\n" // Now we are at the gate.
 "cmp %[lock], %[pass]\n" // See if our pass is any good.
 "jge gate_Enter_skip\n" // Skip if pass >= lock.
 "mov %[lock], %%eax\n" // eax = 1.
 "lock xchg %%eax, %[gate]\n" // Attempt to validate our pass and connect it to the gate.
 "test %%eax, %%eax\n" // Hope for the best.
 "jnz gate_Enter_wait\n" // There is no hope, go back in line.
 "gate_Enter_skip:\n" // We are VIP!
 "add %[lock], %[pass]\n" // Checkin pass like pro!
 : [gate] "=m" (*gate), [pass] "=m" (*pass)
 : [lock] "r" (1)
 : "eax"
 );
}
/*extern*/ inline void
gate_Leave
(gate_Gate *gate, gate_Pass *pass)
{
 asm volatile (
 "add %[checkout], %[pass]\n"
 "js error_abort\n" // Abort, underflow on *pass
 "jnz gate_Leave_skip\n" // Skip next step if I have checked in more then once.
 "mov %[unlock], %%eax\n"
 "lock xchg %%eax, %[gate]\n"
 "test %%eax, %%eax\n"
 "jnz gate_Leave_skip\n" // If zero, abort because we unlocked an unlocked gate!
 "error_abort:\n"
 "call abort\n"
 "gate_Leave_skip:\n" 
 : [gate] "=m" (*gate), [pass] "=m" (*pass)
 : [unlock] "r" (0), [checkout] "r" (-1), [isLast] "r" (1)
 : "eax"
 );
}
// Example use
gate_Gate gate = 0;
__thread gate_Pass exampleUsePass = 0; // notice this uses thread
 // local storage
void exampleUse(int count)
{
 printf("pass=%d\n", exampleUsePass);
 gate_Enter(&gate, &exampleUsePass);
 if (count == 3) {
 // exampleUsePass = 0; // force deadlock.
 }
 if (count < 5) {
 printf("Count = %d\n", count);
 exampleUse(++count);
 }
// uncomment this line to simulate Leaving twice when we owned the gate
//gate_Leave(&gate, &exampleUsePass);
 gate_Leave(&gate, &exampleUsePass);
 printf("pass=%d\n", exampleUsePass);
}
void *doSomeThing(void *arg) {
 // Uncomment lines in this method to force alternate thread to
 // try to leave gate, when it doesn't own it.
 // if(arg == NULL)
 exampleUse(0);
 // gate_Leave(&gate, &exampleUsePass);
 return 0;
}
pthread_t tid[2];
int main(void)
{
 int i = 0;
 int err;
 while(i < 2)
 {
 err = pthread_create(&(tid[i]), NULL, &doSomeThing, i?&err:0);
 if (err != 0)
 printf("\ncan't create thread :[%s]", strerror(err));
 else
 printf("\n Thread created successfully\n");
 i++;
 }
 sleep(5);
 return 0;
}