linux-kernel - Re: [PATCH 03/18] tty: Simplify tty buffer/ldisc interface with helper function

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Date:	Tue, 19 Mar 2013 19:50:36 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Ilya Zykov <linux@...k.ru>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, Min Zhang <mzhang@...sta.com>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/18] tty: Simplify tty buffer/ldisc interface with
 helper function

On Wed, 2013-03-20 at 02:42 +0400, Ilya Zykov wrote:
> On 20.03.2013 0:21, Peter Hurley wrote:
> > Ldisc interface functions must be called with interrupts enabled.
> > Separating the ldisc calls into a helper function simplies the
> > spin lock management.
> > 
> > Update the buffer's read index _after_ the data has been received
> > by the ldisc.
> > 
> 
> Hello Peter!
> It looks good for me.
> I think also we can remove two variables without waste:
> (char_buf), (flag_buf) and use without (&buf->lock)
> (head->char_buf_ptr + head->read), (head->char_buf_ptr + head->read),
> because (head->read) guarded by (TTYP_FLUSHING).

Hi Ilya,
Good to hear from you again.

Yes, I agree, head->read can be safely read and modified here without
owning the buf->lock. And as you correctly point out, there is no need
to make a snapshot of the buf pointers so those locals can be removed.

I'll redo this patch to add both those suggestions. Thanks!

> I have little question about flush_to_ldisc().
> Does can it be multithreaded?
>
> I think yes, because on SMP schedule_work() can work on different CPU paralleled.

Yes, the same work item can now run in parallel on SMP since Tejun Heo
re-did the workqueue implementation on 2.6.36 [Stefan Richter, the
firewire maintainer, recently explained this history to me].

> What do you think about this race condition?
> https://lkml.org/lkml/2011/11/7/98

Yes, that is a possible race condition that could lead to some nasty
results. Good find.

If you want, I could bring that patch into this patchset or you could
re-submit that patch to Greg and I could rebase this patchset on top of
that.

Regards,
Peter Hurley

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