Phil Endecott writes: > Andrew Haley wrote: > > this stuff really should be done by the compiler. > > Yes. I've filed a bug asking for a __sync_lock_test_and_set builtin: > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33413 Surer, but the problem is that for most of the things we want to do (lightweight locks, for example) __sync_lock_test_and_set() doesn't really do what we need: we need compare_and_swap(). That's why the kernel helper is so useful, because it's robust even if we are on pre-ARMv6 hardware. ARM code is like this: /* Atomic compare and exchange. These sequences are not actually atomic; there is a race if *ADDR != OLD_VAL and we are preempted between the two swaps. However, they are very close to atomic, and are the best that a pre-ARMv6 implementation can do without operating system support. LinuxThreads has been using these sequences for many years. */ inline static bool compare_and_swap(volatile obj_addr_t *addr, obj_addr_t old_val, obj_addr_t new_val) { volatile obj_addr_t result, tmp; __asm__ ("\n" "0: ldr %[tmp],[%[addr]]\n" " cmp %[tmp],%[old_val]\n" " movne %[result],#0\n" " bne 1f\n" " swp %[result],%[new_val],[%[addr]]\n" " cmp %[tmp],%[result]\n" " swpne %[tmp],%[result],[%[addr]]\n" " bne 0b\n" " mov %[result],#1\n" "1:" : [result] "=&r" (result), [tmp] "=&r" (tmp) : [addr] "r" (addr), [new_val] "r" (new_val), [old_val] "r" (old_val) : "cc", "memory"); return result; }
