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

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Date:	Tue, 13 Oct 2009 07:39:31 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Nix <nix@...eri.org.uk>, Paul Fulghum <paulkf@...rogate.com>,
	"Justin P. Mattock" <justinmattock@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Boyan <btanastasov@...oo.co.uk>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Ed Tomlinson <edt@....ca>,
	Frédéric L. W. Meunier 
	<fredlwm@...il.com>, OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: [Bug #14388] keyboard under X with 2.6.31

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
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