segmentation fault with cmpxchg in inline asm

Question

I'm writing my_simple_mutex using inline asm. The part of the code below that is commented out works fine, however, the version with cmpxchg terminates with a segfault. I'm using g++ 4.8.2 in cygwin.

void simple_mutex::spin_lock(){
        /*asm ("spin_lock:\n\t"
             "rep; nop;\n\t"
             "lock; bts $0x00, %0;\n\t"
             "jc spin_lock;\n\t"
             :"=m"(lock)
             :"m"(lock)
             :
             );
             */

        asm ("spin_lock:\n\t"
             "rep; nop;\n\t"
             "movl $0x00, %%eax\n\t"
             "movl $0x01, %%edx\n\t"
             "lock; cmpxchg %%edx, %0\n\t"
             "jnz spin_lock;\n\t"
             :"=m"(lock)
             :"m"(lock)
             :
             );

}

The variable lock is of type int . Any ideas what I'm doing wrong?

Answer 1

Possibly the fault is elsewhere, due to the fact that you have forgotten to tell the compiler you modified eax and edx . The fix is to list them as clobbers (the part after the 3rd colon). Unless you are forced to use inline asm, use the atomic builtins instead.