From: Jiri Slaby <jslaby@suse.cz>
To: Ivo Sieben <meltedpianoman@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>,
Greg KH <gregkh@linuxfoundation.org>,
linux-serial@vger.kernel.org, RT <linux-rt-users@vger.kernel.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH] tty: cleanup duplicate functions in tty_buffer
Date: Mon, 24 Sep 2012 10:15:54 +0200 [thread overview]
Message-ID: <506016BA.2070704@suse.cz> (raw)
In-Reply-To: <CAMSQXEHkjWubwNXiCd_bBnUdzQ2M+sa8HPs2KCzwcOOp2dSsBw@mail.gmail.com>
On 09/24/2012 10:02 AM, Ivo Sieben wrote:
> Hi,
>
> 2012/9/20 Alan Cox <alan@linux.intel.com>:
>>
>> So now when the user sets tty->low_latency on these devices the machine
>> crashes ?
>>
>> This is the wrong direction - in fact we have a pile we need to move
>> the other way !
>>
>> If they resolve to the same thing in hard RT patches fine, that's a
>> different question.
>
> Sorry, Allan you are probably right, but I don't get it...
>
> I agree that when the tty->low_latency flag is set on these machines,
> the drivers that were modified by my patch will behave differently:
> they will call the flush_to_ldisc() function directly instead of using
> the work queue. So that indeed introduces different functionality.
>
> But what I don't understand is how this would cause the machine to
> crash? Even when the flush_to_ldisc() function is called from hard IRQ
> context this would cause no problems: the flush_to_ldisc() function
> uses IRQ save spin locks instead of mutexes to protect it's critical
> section. Right?
Yes, but there are deadlocks caused when low_latency is set. Simply
because flush_to_ldisc calls tty->ops->flush_chars from ldisc's
receive_buf. And some drivers take a lock there. But they already hold
the lock from the site where they call tty_schedule_flip from.
> Furthermore the majority of TTY drivers currently already use the
> tty_flip_buffer_push() function.
Yes, but not all call tty_flip_buffer_push correctly. If you set
low_latency for them, they explode.
regards,
--
js
suse labs
next prev parent reply other threads:[~2012-09-24 8:15 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-20 13:53 [PATCH] tty: cleanup duplicate functions in tty_buffer Ivo Sieben
2012-09-20 16:17 ` Alan Cox
2012-09-24 8:02 ` Ivo Sieben
2012-09-24 8:15 ` Jiri Slaby [this message]
2012-09-24 9:25 ` Alan Cox
2012-09-25 11:28 ` Ivo Sieben
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