> But we can do that with __get_user(thread_info->cpu) (very unlikely page > fault protection due to the possibility of CONFIG_DEBUG_PAGEALLOC) and > then validating the cpu. It it's in range, we can use it and verify > whether cpu_rq(cpu)->curr has that thread_info. > > So we can do all that locklessly and optimistically, just going back and > verifying the results later. This is why "thread_info" is actually a > better thing to use than "task_struct" - we can look up the cpu in it with > a simple dereference. We knew the pointer _used_ to be valid, so in any > normal situation, it will never page fault (and if you have > CONFIG_DEBUG_PAGEALLOC and hit a very unlucky race, then performance isn't > your concern anyway: we just need to make the page fault be non-lethal ;) The problem with probe_kernel_address() is that it does lots of operations around the access in the hot path (set_fs, pagefault_disable etc.), so i'm not sure that's a good idea. Sure you can probably do better, but that would involve patching all architectures won't it? Ok I suppose you could make an ARCH_HAS_blabla white list, but that wouldn't be exactly pretty. -Andi -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
