Re: [PATCH 03/18] tty: Simplify tty buffer/ldisc interface withhelper function

From: Peter Hurley
Date: Tue Mar 19 2013 - 19:50:51 EST

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?

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

Peter Hurley

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