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    SubjectRe: kernel oops!


    On Sun, 23 Jan 2005, ierdnah wrote:
    >
    > (gdb) disassemble pty_chars_in_buffer
    ...
    > 0xc02c97a7 <pty_chars_in_buffer+23>: mov 0x28(%edx),%ecx **
    > 0xc02c97aa <pty_chars_in_buffer+26>: test %ecx,%ecx
    > 0xc02c97ac <pty_chars_in_buffer+28>: jne 0xc02c97b6
    > 0xc02c97ae <pty_chars_in_buffer+30>: mov 0x4(%esp,1),%ebx
    > 0xc02c97b2 <pty_chars_in_buffer+34>: add $0x8,%esp
    > 0xc02c97b5 <pty_chars_in_buffer+37>: ret
    > 0xc02c97b6 <pty_chars_in_buffer+38>: mov %edx,(%esp,1)
    > 0xc02c97b9 <pty_chars_in_buffer+41>: call *0x28(%edx) **

    Ahh, indeed. When I compiled this function, it kept 0x28(%edx) in a
    register. Your config/compiler combination does not, so there actually is
    a race condition if somebody changes the function pointer.

    > this is another compiled kernel, but is compiled with the same .config
    > file and same gcc version...because I only have the bzImage, how do I
    > convert it to vmlinux?

    Don't worry, it clearly shows that it's at least possible, and worth
    looking at.

    For testing, a patch like this might get rid of the problem by hiding the
    race (but for all I know, gcc ends up re-loading it anyway) - you might
    want to check the disassembly.

    However, this patch is just for testing, to verify that your problem
    really is that particular race. It's not a proper fix.

    I suspect that pty's should always lock each others line disciplines too,
    not just their "own" side.

    Linus

    ----
    --- 1.32/drivers/char/pty.c 2005-01-10 17:29:36 -08:00
    +++ edited/drivers/char/pty.c 2005-01-23 09:49:16 -08:00
    @@ -149,13 +149,15 @@
    static int pty_chars_in_buffer(struct tty_struct *tty)
    {
    struct tty_struct *to = tty->link;
    + ssize_t (*chars_in_buffer)(struct tty_struct *);
    int count;

    - if (!to || !to->ldisc.chars_in_buffer)
    + /* We should get the line discipline lock for "tty->link" */
    + if (!to || !(chars_in_buffer = to->ldisc.chars_in_buffer))
    return 0;

    /* The ldisc must report 0 if no characters available to be read */
    - count = to->ldisc.chars_in_buffer(to);
    + count = chars_in_buffer(to);

    if (tty->driver->subtype == PTY_TYPE_SLAVE) return count;

    -
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