On Tue, Aug 25, 2015 at 10:58 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >>>>>>> Hi! >>>>>>> >>>>>>> We are working on a dynamic data race detector for the Linux kernel >>>>>>> called KernelThreadSanitizer (ktsan) >>>>>>> (https://github.com/google/ktsan/wiki). >>>>>>> >>>>>>> While booting the kernel (upstream revision 21bdb584af8c) we got a report: >>>>>>> >>>>>>> ================================================================== >>>>>>> ThreadSanitizer: data-race in uart_ioctl >>>>>>> >>>>>>> Read of size 8 by thread T424 (K971): >>>>>>> [<ffffffff81673c56>] uart_ioctl+0x36/0x11e0 >>>>>>> drivers/tty/serial/serial_core.c:1216 >>>>>>> [<ffffffff81643802>] tty_ioctl+0x4f2/0x11d0 drivers/tty/tty_io.c:2924 >>>>>>> [< inlined >] do_vfs_ioctl+0x44a/0x750 vfs_ioctl fs/ioctl.c:43 >>>>>>> [<ffffffff8127b0ca>] do_vfs_ioctl+0x44a/0x750 fs/ioctl.c:607 >>>>>>> [< inlined >] SyS_ioctl+0x79/0xa0 SYSC_ioctl fs/ioctl.c:622 >>>>>>> [<ffffffff8127b449>] SyS_ioctl+0x79/0xa0 fs/ioctl.c:613 >>>>>>> [<ffffffff81eae0ae>] entry_SYSCALL_64_fastpath+0x12/0x71 >>>>>>> arch/x86/entry/entry_64.S:186 >>>>>>> DBG: cpu = ffff88063fc1fe68 >>>>>>> DBG: cpu id = 0 >>>>>>> >>>>>>> Previous write of size 8 by thread T422 (K970): >>>>>>> [<ffffffff816737ef>] uart_open+0x12f/0x220 >>>>>>> drivers/tty/serial/serial_core.c:1629 >>>>>>> [<ffffffff81645be2>] tty_open+0x192/0x8f0 drivers/tty/tty_io.c:2105 >>>>>>> [<ffffffff812628fc>] chrdev_open+0x13c/0x290 fs/char_dev.c:388 >>>>>>> [<ffffffff812582fc>] do_dentry_open+0x3ac/0x550 fs/open.c:736 >>>>>>> [<ffffffff81259d68>] vfs_open+0xb8/0xe0 fs/open.c:853 >>>>>>> [< inlined >] path_openat+0x81c/0x2440 do_last fs/namei.c:3163 >>>>>>> [<ffffffff81272f1c>] path_openat+0x81c/0x2440 fs/namei.c:3295 >>>>>>> [<ffffffff8127656a>] do_filp_open+0xfa/0x170 fs/namei.c:3330 >>>>>>> [<ffffffff8125a243>] do_sys_open+0x183/0x2b0 fs/open.c:1025 >>>>>>> [< inlined >] SyS_open+0x35/0x50 SYSC_open fs/open.c:1043 >>>>>>> [<ffffffff8125a3a5>] SyS_open+0x35/0x50 fs/open.c:1038 >>>>>>> [<ffffffff81eae0ae>] entry_SYSCALL_64_fastpath+0x12/0x71 >>>>>>> arch/x86/entry/entry_64.S:186 >>>>>>> DBG: cpu = ffff88063fd1fe68 >>>>>>> >>>>>>> DBG: addr: ffff8801d2a0ce88 >>>>>>> DBG: first offset: 0, second offset: 0 >>>>>>> DBG: T424 clock: {T424: 211057, T422: 275728} >>>>>>> DBG: T422 clock: {T422: 275819} >>>>>>> ================================================================== >>>>>>> >>>>>>> It seems that one thread reads and uses tty->driver_data while it's >>>>>>> being initialized in another one. The second thread holds port->mutex, >>>>>>> but the first one does a few accesses to tty->driver_data before >>>>>>> locking it. >>>>>>> >>>>>>> Could you confirm if this is a real race? >>>>>> >>>>>> Although I don't understand what triggers ktsan to signal a race >>>>>> condition, this doesn't appear to be an actual race. >>>>>> >>>>>> For an ioctl() syscall to act on any given tty requires a successful >>>>>> open() syscall to have nearly completed (do_sys_open() => fd_install() >>>>>> initializes the file descriptor; ioctl() => fdget() derefs the descriptor). >>>>>> >>>>>> Perhaps what's tripping the race detection is that 2nd and subsequent >>>>>> opens also (redundantly) write the same values as from the first open? >>>>> >>>>> Since we use a fuzzer, yes, it is possible that open is called twice. >>>> >>>> Oh, no, sorry, this happens during booting. >>>> The race is on tty_struct, which is probably shared between several >>>> file descriptors. >>> >>> Yep, but there is 1:1 correspondence between tty_struct and uart_state; >>> so once the first open() initializes tty->driver_data, subsequent opens >>> are just re-writing the same value for tty->driver_data. >>> >>> Is ktsan just triggering on the fact there is a memory write, without >>> checking the value has changed? >> >> Yes, that is correct. Ktsan is based on C memory model which says that >> any write racing with other memory access leads to undefined behavior. > > The Linux kernel largely ignores the C memory model definition, and > relies on practical compiler behavior. > > So-called 'data races' are common in kernel code. Yeah, I know. But the problem is that with the kernel model it is not possible to automatically find real harmful race conditions, so we have to stick with something that allows that. >> As far as I see the setup is quite complex and not just writes to >> tty->driver_data. > > Yes, vfs => tty => serial core => uart driver is a (necessarily) complicated > stack. > >> For example, it also calls uart_startup which does >> set_bit(TTY_IO_ERROR, &tty->flags) on the tty. Can't that lead to some >> issues? > > No. > >> E.g. we set TTY_IO_ERROR, but since tty is already used by >> other threads, operations on the tty in other threads will spuriously >> fail. > > See below. > >> 195 static int uart_startup(struct tty_struct *tty, struct uart_state *state, >> 196 int init_hw) >> 197 { >> 198 struct tty_port *port = &state->port; >> 199 int retval; >> 200 >> 201 if (port->flags & ASYNC_INITIALIZED) >> 202 return 0; > > Subsequent opens return success here (note below how the ASYNCB_INITIALIZED bit > is set in tty->flags if uart_port_startup() returned success from the earlier > open ?) Ah, OK, we will mark this data race as intentional then. Thank you. -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
