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SubjectRe: [PATCH v2] n_tty: Fix read_buf race condition, increment read_head after pushing data
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On Wed, Nov 12, 2014 at 12:53 PM, Måns Rullgård <mans@mansr.com> wrote:
> Christian Riesch <christian.riesch@omicron.at> writes:
>
>> On Tue, Nov 11, 2014 at 2:04 PM, Måns Rullgård <mans@mansr.com> wrote:
>>> Christian Riesch <christian.riesch@omicron.at> 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.

Ok. I will send a new patch without the comment.
Thanks, Christian
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