Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: patch "TTY: remove tty_locked" added to tty tree | Date | Wed, 24 Aug 2011 16:35:13 +0200 |
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On Wednesday 24 August 2011, Jiri Slaby wrote: > On 08/24/2011 01:20 PM, Arnd Bergmann wrote: > > It's not clear to me what state->mutex protects in the serial_core, but > > it has been around forever (used to be called state->sem) > > It was actually moved in uart_close back in 2003. Formerly (when there > was only a coarse grained port_sem) it was right before uart_shutdown. > But there were some flags to handle some races. I'm not sure whether the > flags protected any race here though.
ok
> > and is held in > > all uart functions, which is at least consistent. IIRC what Alan's plan > > for this was, uart_close should eventually get changed to use > > tty_port_close_start or even tty_port_close. Maybe the time for that has > > come now, lacking better alternatives? > > Yes, I have such a patch in my queue. But it's not easy to get there. > You may take a look at: > http://decibel.fi.muni.cz/gitweb/?p=linux.git;a=shortlog;h=refs/heads/devel > > But it's still far from ready. And yet, in the queue, I still have > port->mutex locked before tty_port_close_start like it is now.
Ah, right. I still don't see why the port->mutex is or is not needed there, and I think that's the main issue.
By comparison, getting *_wait_until_sent to be called without BTM seems easy -- we know that all callers from ->close() hold it, while the ones from ->ioctl() don't. We could have a helper like
void tty_wait_until_sent_from_close(struct tty_struct *tty, long timeout) { tty_unlock(); /* tty->ops->close holds the BTM, drop it while waiting */ tty_wait_until_sent(tty, timeout); tty_lock(); }
to deal with that, if only we can sort the lock ordering with port->mutex.
BTW, I saw that the three m68k serial port drivers (amiserial, 68328, 68360) all call *_wait_until_sent with interrupts disabled, which is even more broken.
Arnd
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