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 -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
