linux-kernel - Re: UART/TTY console deadlock

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Message-ID: <20200703105302.GC182102@jagdpanzerIV.localdomain>
Date:   Fri, 3 Jul 2020 19:53:02 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tony Lindgren <tony@...mide.com>,
        Petr Mladek <pmladek@...e.com>,
        Raul Rangel <rrangel@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kurt@...utronix.de, "S, Shirish" <Shirish.S@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        John Ogness <john.ogness@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: UART/TTY console deadlock

On (20/07/02 11:20), Andy Shevchenko wrote:
> 
> 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.

Hmm. Would different IRQ handlers use different UART port structures? ->irq
might be the only thing they share. Even if they use the same port, we
keep IRQs disabled on local CPU, and UART port locked. To unlock the UART
port during IRQ, the handler (on another CPU) first needs to acquire the
port->lock, which is locked by serial8250_do_startup().

	-ss