Potential data race in uart_ioctl

From: Andrey Konovalov
Date: Tue Aug 25 2015 - 08:17:33 EST


We are working on a dynamic data race detector for the Linux kernel
called KernelThreadSanitizer (ktsan)

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
[<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
DBG: cpu = ffff88063fc1fe68
DBG: cpu id = 0

Previous write of size 8 by thread T422 (K970):
[<ffffffff816737ef>] uart_open+0x12f/0x220
[<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
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?

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