Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 7 Jun 2011 19:44:48 -0700 | Subject | Re: tty breakage in X (Was: tty vs workqueue oddities) |
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On Mon, Jun 6, 2011 at 7:24 AM, Guillaume Chazarain <guichaz@gmail.com> wrote: > > After reverting http://git.kernel.org/linus/a5660b4 "tty: fix endless > work loop when the buffer fills up" I cannot reproduce the hangs on > SMP anymore but it brings back the busy loop on UP.
Hmm. The n_tty layer has some rather distressing locking, and doesn't lock "tty->receive_room" changes at all, for example (and uses multiple locks for some other things).
It may well be that there is some SMP race there. The n_tty line discipline has its own locking for its counts, and the tty buffer code has its own locking, and "receive_room" kind o fends up being in the middle between them.
The sad part is that the patch that made receive_buf() return the amout of bytes received was actually trying to do the right thing, it just did it entirely in the wrong way (re-introducing the crazy re-arming of the workqueue from within itself, and using all the wrong sign issues).
I'd love to get rid of receive_room entirely - and just letting the tty line discipline handler say how much it actually received. in other words, having receive_buf() just tell us how much it used, and not looking at receive_room in the caller is absolutely the right thing.
It just needs to be done properly.
Linus
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