From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>, Petr Mladek <pmladek@suse.com>,
Raul Rangel <rrangel@google.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
kurt@linutronix.de, "S, Shirish" <Shirish.S@amd.com>,
Peter Zijlstra <peterz@infradead.org>,
John Ogness <john.ogness@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: UART/TTY console deadlock
Date: Thu, 2 Jul 2020 11:20:08 +0300 [thread overview]
Message-ID: <20200702082008.GE3703480@smile.fi.intel.com> (raw)
In-Reply-To: <20200702054033.GC3450@jagdpanzerIV.localdomain>
On Thu, Jul 02, 2020 at 02:40:33PM +0900, Sergey Senozhatsky wrote:
> On (20/07/02 14:12), Sergey Senozhatsky wrote:
> > From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > On (20/06/30 11:02), Tony Lindgren wrote:
...
> > I think we can do it. serial8250_do_startup() and irq handler take
> > port->lock, so they should be synchronized.
>
> Hmm, hold on. Why does it disable IRQ in the first place? IRQ handlers
> should grab the port->lock. So if there is already running IRQ, then
> serial8250_do_startup() will wait until IRQ handler unlocks the port->lock.
> If serial8250_do_startup() grabs the port->lock first, then IRQ will wait
> for serial8250_do_startup() to unlock it. serial8250_do_startup() does
> not release the port->unlock until its done:
>
> spin_lock_irqsave(&port->lock, flags);
>
> wait_for_xmitr(up, UART_LSR_THRE);
> serial_port_out_sync(port, UART_IER, UART_IER_THRI);
> udelay(1); /* allow THRE to set */
> iir1 = serial_port_in(port, UART_IIR);
> serial_port_out(port, UART_IER, 0);
> serial_port_out_sync(port, UART_IER, UART_IER_THRI);
> udelay(1); /* allow a working UART time to re-assert THRE */
> iir = serial_port_in(port, UART_IIR);
> serial_port_out(port, UART_IER, 0);
>
> spin_unlock_irqrestore(&port->lock, flags);
>
> so IRQ will not see the inconsistent device state.
>
> What exactly is the purpose of disable_irq_nosync()? Can we just remove
> disable_irq_nosync()/enable_irq() instead? Are there any IRQ handlers
> that don't acquire the port->lock?
I didn't look into this deeply, but my understanding that this is something for
special case when you have several UART ports sharing the IRQ (multi-port card)
and IRQ even maybe undesirable b/c it will confuse real IRQ handler. I don't
remember details, but AFAIR IRQ handler does a busyloop to service as much as
possible and in between it may release the lock (again, multi-port UART cards),
that's why we better avoid IRQ event in the first place.
But it's my pure speculation.
--
With Best Regards,
Andy Shevchenko
next prev parent reply other threads:[~2020-07-02 8:20 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-22 17:30 UART/TTY console deadlock Raul Rangel
2020-06-22 17:37 ` Andy Shevchenko
2020-06-30 3:58 ` Sergey Senozhatsky
2020-06-30 10:21 ` Petr Mladek
2020-06-30 10:55 ` Sergey Senozhatsky
2020-06-30 11:40 ` Andy Shevchenko
2020-06-30 12:22 ` Petr Mladek
2020-06-30 13:05 ` Sergey Senozhatsky
2020-06-30 18:02 ` Tony Lindgren
2020-07-01 6:44 ` S, Shirish
2020-07-02 3:48 ` S, Shirish
2020-07-02 6:11 ` Greg Kroah-Hartman
2020-07-02 6:14 ` S, Shirish
2020-07-02 6:34 ` Sergey Senozhatsky
2020-07-02 7:11 ` Greg Kroah-Hartman
2020-07-02 5:12 ` Sergey Senozhatsky
2020-07-02 5:40 ` Sergey Senozhatsky
2020-07-02 8:20 ` Andy Shevchenko [this message]
2020-07-03 10:53 ` Sergey Senozhatsky
2020-07-04 11:37 ` Andy Shevchenko
2020-07-02 16:05 ` Tony Lindgren
2020-07-03 10:32 ` Sergey Senozhatsky
2020-07-04 11:35 ` Andy Shevchenko
2020-07-04 11:59 ` Andy Shevchenko
2020-07-06 11:31 ` Kurt Kanzenbach
2020-07-06 14:43 ` Sergey Senozhatsky
2020-07-08 7:40 ` Kurt Kanzenbach
2020-07-08 8:07 ` Sergey Senozhatsky
2020-07-08 9:52 ` Petr Mladek
2020-07-09 13:22 ` Sergey Senozhatsky
2020-07-14 16:16 ` Raul Rangel
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