On 08/25/2015 02:38 PM, Dmitry Vyukov wrote: > On Tue, Aug 25, 2015 at 8:32 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >> On Tue, Aug 25, 2015 at 8:26 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >>> On 08/25/2015 08:17 AM, Andrey Konovalov 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? Regards, Peter Hurley -- 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
