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    Subject[PATCH 06/13] tty: never hold BTM while getting tty_mutex
    Date
    tty_mutex is never taken with the BTM held, except for
    two corner cases that are worked around here.
    We give up the BTM before calling tty_release() in the
    error path of tty_open().
    Similarly, we reorder the locking in ptmx_open()
    to get tty_mutex before the BTM.

    Signed-off-by: Arnd Bergmann <arnd@arndb.de>
    ---
    drivers/char/pty.c | 22 ++++++++--------------
    drivers/char/tty_io.c | 13 +++++++------
    2 files changed, 15 insertions(+), 20 deletions(-)

    diff --git a/drivers/char/pty.c b/drivers/char/pty.c
    index dbb144b..889de89 100644
    --- a/drivers/char/pty.c
    +++ b/drivers/char/pty.c
    @@ -626,7 +626,7 @@ static const struct tty_operations pty_unix98_ops = {
    * allocated_ptys_lock handles the list of free pty numbers
    */

    -static int __ptmx_open(struct inode *inode, struct file *filp)
    +static int ptmx_open(struct inode *inode, struct file *filp)
    {
    struct tty_struct *tty;
    int retval;
    @@ -635,11 +635,14 @@ static int __ptmx_open(struct inode *inode, struct file *filp)
    nonseekable_open(inode, filp);

    /* find a device that is not in use. */
    + tty_lock();
    index = devpts_new_index(inode);
    + tty_unlock();
    if (index < 0)
    return index;

    mutex_lock(&tty_mutex);
    + tty_lock();
    tty = tty_init_dev(ptm_driver, index, 1);
    mutex_unlock(&tty_mutex);

    @@ -657,24 +660,15 @@ static int __ptmx_open(struct inode *inode, struct file *filp)
    goto out1;

    retval = ptm_driver->ops->open(tty, filp);
    - if (!retval)
    - return 0;
    + if (retval)
    + tty_release(inode, filp);
    out1:
    - tty_release(inode, filp);
    + tty_unlock();
    return retval;
    out:
    devpts_kill_index(inode, index);
    - return retval;
    -}
    -
    -static int ptmx_open(struct inode *inode, struct file *filp)
    -{
    - int ret;
    -
    - tty_lock();
    - ret = __ptmx_open(inode, filp);
    tty_unlock();
    - return ret;
    + return retval;
    }

    static struct file_operations ptmx_fops;
    diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
    index 82c723a..e1b2fca 100644
    --- a/drivers/char/tty_io.c
    +++ b/drivers/char/tty_io.c
    @@ -1871,21 +1871,22 @@ got_driver:
    printk(KERN_DEBUG "error %d in opening %s...", retval,
    tty->name);
    #endif
    + tty_unlock(); /* need to call tty_release without BTM */
    tty_release(inode, filp);
    - if (retval != -ERESTARTSYS) {
    - tty_unlock();
    + if (retval != -ERESTARTSYS)
    return retval;
    - }
    - if (signal_pending(current)) {
    - tty_unlock();
    +
    + if (signal_pending(current))
    return retval;
    - }
    +
    schedule();
    /*
    * Need to reset f_op in case a hangup happened.
    */
    + tty_lock();
    if (filp->f_op == &hung_up_tty_fops)
    filp->f_op = &tty_fops;
    + tty_unlock();
    goto retry_open;
    }
    tty_unlock();
    --
    1.7.0.4


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