pty_write: don't do a tty_wakeup() when the buffers are full
commit 202c4675 upstream. Commit ac89a917 ("pty: don't limit the writes to 'pty_space()' inside 'pty_write()'") removed the pty_space() checking, in order to let the regular tty buffer code limit the buffering itself. That was all good, but as a subtle side effect it meant that we'd be doing a tty_wakeup() even in the case where the buffers were all filled up, and didn't actually make any progress on the write. Which sounds innocuous, but it interacts very badly with the ppp_async code, which has an infinite loop in ppp_async_push() that tries to push out data to the tty. When we call tty_wakeup(), that loop ends up thinking that progress was made (see the subtle interactions between XMIT_WAKEUP and 'tty_stuffed' for details). End result: one unhappy ppp user. Fixed by noticing when tty_insert_flip_string() didn't actually do anything, and then not doing any more processing (including, very much not calling tty_wakeup()). Bisected-and-tested-by: Peter Volkov <pva@gentoo.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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