Something

".balign 8\n"
".globl mutex_lock\n"
"mutex_lock:\n"

  "mov 4(%esp), %eax;"
"1:"
  "bt $0, (%eax);" 
  "jc 1b;"
"2:"	    
  "lock; bts $0, (%eax);" // atomic test for bit 0
  "jc 1b;"
  "ret;"
);