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This is my second attempt to make simple lock using extended assembly. See https://stackoverflow.com/questions/37241553/locks-around-memory-manipulation-via-inline-assembly/37246263

The code:

static inline void atomic_open(volatile int *gate)
{
 asm volatile (
 "jmp check\n"
 "wait:\n" // Set spawning point.
 "pause\n" // Stroke your beard. (If you have one.)
 "check:\n"
 "cmp %[open], %[gate]\n" // Check if gate is open.
 "jne wait\n" // If it isn't open, wait. If it is open, go through the gate.
 "lock xadd %[lock], %[gate]\n" // In case you where not alone entering the gate, at least try to be the first one to put the lock.
 "cmp %[lock], %[open]\n" // Check to see you where the first one who put the lock.
 // "If not, consider thug life..."
 "jne wait\n" // If you didn't win, respawn and try again later.
 : [gate] "=m" (*gate)
 : [lock] "r" (1), [open] "r" (0)
 );
}
static inline void atomic_close(volatile int *gate)
{
 asm volatile (
 "pause\n"
 "lock xchg %[lock], %[gate]\n"
 : [gate] "=m" (*gate)
 : [lock] "r" (0)
 );
}
// Usage, example.
volatile int atomic_gate_memory = 0;
void *mymalloc(size_t size)
{
 atomic_open(&atomic_gate_memory);
 void *ptr = malloc(size);
 atomic_close(&atomic_gate_memory);
 return ptr;
}

The question: Will atomic_[open/close] make mymalloc both threadsafe and reentrant?

• If no, what is wrong?
• If yes, it is still wrong; isn't it?... Give me a good rant about what to consider, what is missing or about better approach. If you want to suggest libraries, please restrict your self to C. I am not experienced enough to bind C++ stuff to other languages, so I often can't use the good stuff over there :'(