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authorLinus Torvalds <torvalds@linux-foundation.org>2009-10-14 08:59:49 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2009-10-14 15:09:52 -0700
commitc8e33141911bf8fe87dc6c92793b9a59b2be0130 (patch)
tree7b3561169050d242469df55ed338e752c9a35c48 /kernel/workqueue.c
parent80f506918fdaaca6b574ba931536a58ce015c7be (diff)
tty: Make flush_to_ldisc() locking more robust
The locking logic in this function is extremely subtle, and it broke when we started doing potentially concurrent 'flush_to_ldisc()' calls in commit e043e42bdb66885b3ac10d27a01ccb9972e2b0a3 ("pty: avoid forcing 'low_latency' tty flag"). The code in flush_to_ldisc() used to set 'tty->buf.head' to NULL, with the intention that this would then cause any other concurrent calls to not do anything (locking note: we have to drop the buf.lock over the call to ->receive_buf that can block, which is why we can have concurrency here at all in the first place). It also used to set the TTY_FLUSHING bit, which would then cause any concurrent 'tty_buffer_flush()' to not free all the tty buffers and clear 'tty->buf.tail'. And with 'buf.head' being NULL, and 'buf.tail' being non-NULL, new data would never touch 'buf.head'. Does that sound a bit too subtle? It was. If another concurrent call to 'flush_to_ldisc()' were to come in, the NULL buf.head would indeed cause it to not process the buffer list, but it would still clear TTY_FLUSHING afterwards, making the buffer protection against 'tty_buffer_flush()' no longer work. So this clears it all up. We depend purely on TTY_FLUSHING for handling re-entrancy, and stop playing games with the buffer list entirely. In fact, the buffer list handling is now robust enough that we could probably stop doing the whole "protect against 'tty_buffer_flush()'" thing entirely. However, Alan also points out that we would probably be better off simplifying the locking even further, and just take the tty ldisc_mutex around all the buffer flushing calls. That seems like a good idea, but in the meantime this is a conceptually minimal fix (with the patch itself being bigger than required just to clean the code up and make it readable). This fixes keyboard trouble under X: http://bugzilla.kernel.org/show_bug.cgi?id=14388 Reported-and-tested-by: Frédéric Meunier <fredlwm@gmail.com> Reported-and-tested-by: Boyan <btanastasov@yahoo.co.uk> Cc: Alan Cox <alan@lxorguk.ukuu.org.uk> Cc: Paul Fulghum <paulkf@microgate.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/workqueue.c')
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