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Subject: Re: Potential data race in uart_ioctl
To: Andrey Konovalov <andreyknvl@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
References: <CAAeHK+xPOrm4JMjmRL4Er+LU7Auh_+rSnNcbWniPOoQFMZaqfw@mail.gmail.com>
Cc: Jiri Slaby <jslaby@suse.cz>, linux-serial@vger.kernel.org,
        linux-kernel@vger.kernel.org, Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Kostya Serebryany <kcc@google.com>
From: Peter Hurley <peter@hurleysoftware.com>
Message-ID: <55DCB365.2060501@hurleysoftware.com>
Date: Tue, 25 Aug 2015 14:26:45 -0400
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Hi Andrey,

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?

Regards,
Peter Hurley
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