Re: [Bug #14388] keyboard under X with 2.6.31

Linus Torvalds
2009-10-13T14:39:31Z

On Tue, 13 Oct 2009, Alan Cox wrote:
>
> There is a simple reason the locking is sufficient. If you can call the
> function from two places at once in your serial driver at the same you've
> scrambled the data order so you've already lost.

Umm. No, Alan.

You also can race with:

- whoever is _reading_ the buffer, and due to memory ordering may see the
update to the buffer length _before_ it actually sees the data itself.
That spinlock does all the memory ordering too.

- scrambling the data order with two writers is certainly less annoying
than potentially screwing up ->used entirely, and having the memcpy's
overflow the buffer. Both writers may have decided that there is enough
room for each one - but that does not mean that there is enough room
for _both_.

Now, I do agree that generally there should be locking at a higher level,
and you should never see two concurrent writers. But even if the locking
is only for reading, the old locking is simply _wrong_.

> > pointless: they then call tty_insert_flip_string(), which means that the
> > tty_buffer_request_room() call was totally redundant ]
>
> It's a performance tweak. With a 3G USB modem or similar device running
> at 20Mbits or more being able to generate one allocation per chunk
> received for DMA made a measurable performance difference on some
> platforms.

Have you even _read_ the code, Alan?

It's not a f*cking performance tweak, and you're ludicrous to claim it is.
It's pointless, and it's making the code _slower_ rather than faster.

Lookie here, Alan - the common sequence is crap like this:

tty_buffer_request_room(tty, buf->size);
tty_insert_flip_string(tty, buf->base, buf->size);

and anybody who claims that is a "performance tweak" doesn't know what the
hell he is talking about.

Look again.

The first thing that tty_insert_flup_string() does is to re-do the same
tty_buffer_request_room() call.

Performance tweak? No. Most of them are stupid, pointless, and worthless.
Many of them do it for a single character too.

Not all, no. One or two seem to do one tty_buffer_request_room() call, and
then some one-byte-at-a-time thing, but quite frankly, those are sure as
hell not going to push lots of data quickly that way either.

Maybe there is some driver where there's a point to it, but from a quick
grep, I couldn't find any.

Linus