From: David Vrabel Date: Fri, 19 Jul 2013 14:51:58 +0000 (+0100) Subject: xen/evtchn: avoid a deadlock when unbinding an event channel X-Git-Tag: firefly_0821_release~6453^2~1242 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=3d63d1e0fe8aeed27f6ef38fa6eaf518dee1bbab;p=firefly-linux-kernel-4.4.55.git xen/evtchn: avoid a deadlock when unbinding an event channel commit 179fbd5a45f0d4034cc6fd37b8d367a3b79663c4 upstream. Unbinding an event channel (either with the ioctl or when the evtchn device is closed) may deadlock because disable_irq() is called with port_user_lock held which is also locked by the interrupt handler. Think of the IOCTL_EVTCHN_UNBIND is being serviced, the routine has just taken the lock, and an interrupt happens. The evtchn_interrupt is invoked, tries to take the lock and spins forever. A quick glance at the code shows that the spinlock is a local IRQ variant. Unfortunately that does not help as "disable_irq() waits for the interrupt handler on all CPUs to stop running. If the irq occurs on another VCPU, it tries to take port_user_lock and can't because the unbind ioctl is holding it." (from David). Hence we cannot depend on the said spinlock to protect us. We could make it a system wide IRQ disable spinlock but there is a better way. We can piggyback on the fact that the existence of the spinlock is to make get_port_user() checks be up-to-date. And we can alter those checks to not depend on the spin lock (as it's protected by u->bind_mutex in the ioctl) and can remove the unnecessary locking (this is IOCTL_EVTCHN_UNBIND) path. In the interrupt handler we cannot use the mutex, but we do not need it. "The unbind disables the irq before making the port user stale, so when you clear it you are guaranteed that the interrupt handler that might use that port cannot be running." (from David). Hence this patch removes the spinlock usage on the teardown path and piggybacks on disable_irq happening before we muck with the get_port_user() data. This ensures that the interrupt handler will never run on stale data. Signed-off-by: David Vrabel Signed-off-by: Konrad Rzeszutek Wilk [v1: Expanded the commit description a bit] Signed-off-by: Jonghwan Choi Signed-off-by: Greg Kroah-Hartman --- diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c index 45c8efaa6b3e..34924fb9d02a 100644 --- a/drivers/xen/evtchn.c +++ b/drivers/xen/evtchn.c @@ -377,18 +377,12 @@ static long evtchn_ioctl(struct file *file, if (unbind.port >= NR_EVENT_CHANNELS) break; - spin_lock_irq(&port_user_lock); - rc = -ENOTCONN; - if (get_port_user(unbind.port) != u) { - spin_unlock_irq(&port_user_lock); + if (get_port_user(unbind.port) != u) break; - } disable_irq(irq_from_evtchn(unbind.port)); - spin_unlock_irq(&port_user_lock); - evtchn_unbind_from_user(u, unbind.port); rc = 0; @@ -488,26 +482,15 @@ static int evtchn_release(struct inode *inode, struct file *filp) int i; struct per_user_data *u = filp->private_data; - spin_lock_irq(&port_user_lock); - - free_page((unsigned long)u->ring); - for (i = 0; i < NR_EVENT_CHANNELS; i++) { if (get_port_user(i) != u) continue; disable_irq(irq_from_evtchn(i)); - } - - spin_unlock_irq(&port_user_lock); - - for (i = 0; i < NR_EVENT_CHANNELS; i++) { - if (get_port_user(i) != u) - continue; - evtchn_unbind_from_user(get_port_user(i), i); } + free_page((unsigned long)u->ring); kfree(u->name); kfree(u);