On Wed, 7 Jan 2009 22:37:40 +0100 Andi Kleen <a...@firstfloor.org> wrote:
> > 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. probe_kernel_address() isn't tooooo bad - a few reads and writes into the task_struct and thread_struct. And we're on the slow, contended path here anyway.. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
