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    SubjectRe: hci_ldsic nested locking problem
    On Thu, Mar 20, 2014 at 12:16:22PM -0500, Felipe Balbi wrote:
    > On Thu, Mar 20, 2014 at 04:42:16PM +0000, Alan Cox wrote:
    > > On Thu, 2014-03-20 at 11:34 -0500, Felipe Balbi wrote:
    > > > Hi,
    > > >
    > > > when 8250 driver calls uart_write_wakeup(), the tty port lock is already
    > > > taken. hci_ldisc.c's implementation of ->write_wakeup() calls
    > > > tty->ops->write() to actually send the characters, but that call will
    > > > try to acquire the same port lock again.
    > > >
    > > > Looking at other line disciplines that looks like a bug in hci_ldisc.c.
    > > > Am I correct to assume that ->write_wakeup() is supposed to *just*
    > > > wakeup the bottom half so we handle ->write() in another context ?
    > > >
    > > > Is it legal to call tty->ops->write() from within ->write_wakeup() ?
    > >
    > > It isn't because you might send all the bytes and go
    > >
    > > write
    > > write_wakeup
    > > write
    > > write wakeup
    > > ...
    > >
    > > and recurse

    then we need updates to Documentation:

    Documentation/serial/tty.txt::

    | Driver Side Interfaces:
    |
    | receive_buf() - Hand buffers of bytes from the driver to the ldisc
    | for processing. Semantics currently rather
    | mysterious 8(
    |
    | write_wakeup() - May be called at any point between open and close.
    | The TTY_DO_WRITE_WAKEUP flag indicates if a call
    | is needed but always races versus calls. Thus the
    | ldisc must be careful about setting order and to
    | handle unexpected calls. Must not sleep.
    |
    | The driver is forbidden from calling this directly
    | from the ->write call from the ldisc as the ldisc
    | is permitted to call the driver write method from
    | this function. In such a situation defer it.

    documentation says ldisc is allowed to call ->write() from
    ->write_wakeup(). huh ?

    --
    balbi
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