Re: [PATCH v2] n_tty: Fix read_buf race condition, increment read_head after pushing data

[Måns Rullgård]

Christian Riesch <christian.riesch@xxxxxxxxxx> writes:

> On Tue, Nov 11, 2014 at 2:04 PM, Måns Rullgård <mans@xxxxxxxxx> wrote:
>> Christian Riesch <christian.riesch@xxxxxxxxxx> writes:
> [...]>> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
>>> index 2e900a9..b09f326 100644
>>> --- a/drivers/tty/n_tty.c
>>> +++ b/drivers/tty/n_tty.c
>>> @@ -321,7 +321,9 @@ static void n_tty_check_unthrottle(struct tty_struct *tty)
>>>
>>>  static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
>>>  {
>>> -     *read_buf_addr(ldata, ldata->read_head++) = c;
>>> +     *read_buf_addr(ldata, ldata->read_head) = c;
>>> +     /* increment read_head _after_ placing the character in the buffer */
>>> +     ldata->read_head++;
>>>  }
>>
>> Is that comment really necessary?
>
> No, I am pretty sure that removing the comment would not break the code ;-)
>
> I just thought it would be good to have some kind of reminder here.
> Otherwise someone may think: Hey, it would be a good idea to do the
> increment right in the first line. And submit a patch for it.

The intent all along was to increment after the write.  Nobody needs
reminding of that.  The problem was a misunderstanding of when the
post-increment takes effect.  As much as we'd like for everybody to have
a thorough knowledge of C, a random tty driver doesn't seem the place to
educate them.

-- 
Måns Rullgård
mans@xxxxxxxxx
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