From: Linus Torvalds Date: Sun, 7 Feb 2010 18:11:23 +0000 (-0800) Subject: Fix race in tty_fasync() properly X-Git-Tag: v2.6.27.46~43 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=24fce8f6a79db04afef0c6118f6ecdcfec12ffc4;p=pandora-kernel.git Fix race in tty_fasync() properly commit 80e1e823989ec44d8e35bdfddadbddcffec90424 upstream. This reverts commit 703625118069 ("tty: fix race in tty_fasync") and commit b04da8bfdfbb ("fnctl: f_modown should call write_lock_irqsave/ restore") that tried to fix up some of the fallout but was incomplete. It turns out that we really cannot hold 'tty->ctrl_lock' over calling __f_setown, because not only did that cause problems with interrupt disables (which the second commit fixed), it also causes a potential ABBA deadlock due to lock ordering. Thanks to Tetsuo Handa for following up on the issue, and running lockdep to show the problem. It goes roughly like this: - f_getown gets filp->f_owner.lock for reading without interrupts disabled, so an interrupt that happens while that lock is held can cause a lockdep chain from f_owner.lock -> sighand->siglock. - at the same time, the tty->ctrl_lock -> f_owner.lock chain that commit 703625118069 introduced, together with the pre-existing sighand->siglock -> tty->ctrl_lock chain means that we have a lock dependency the other way too. So instead of extending tty->ctrl_lock over the whole __f_setown() call, we now just take a reference to the 'pid' structure while holding the lock, and then release it after having done the __f_setown. That still guarantees that 'struct pid' won't go away from under us, which is all we really ever needed. Reported-and-tested-by: Tetsuo Handa Acked-by: Greg Kroah-Hartman Acked-by: Américo Wang Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- Reading git-diff-tree failed