Messages in this thread | | | Date | Tue, 10 Feb 2015 15:41:57 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH v2] ARM: Don't use complete() during __cpu_die |
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On Mon, Feb 09, 2015 at 05:24:08PM -0800, Stephen Boyd wrote: > On 02/05/15 08:11, Russell King - ARM Linux wrote: > > On Thu, Feb 05, 2015 at 06:29:18AM -0800, Paul E. McKenney wrote: > >> Works for me, assuming no hidden uses of RCU in the IPI code. ;-) > > Sigh... I kind'a new it wouldn't be this simple. The gic code which > > actually raises the IPI takes a raw spinlock, so it's not going to be > > this simple - there's a small theoretical window where we have taken > > this lock, written the register to send the IPI, and then dropped the > > lock - the update to the lock to release it could get lost if the > > CPU power is quickly cut at that point. > > Hm.. at first glance it would seem like a similar problem exists with > the completion variable. But it seems that we rely on the call to > complete() fom the dying CPU to synchronize with wait_for_completion() > on the killing CPU via the completion's wait.lock. > > void complete(struct completion *x) > { > unsigned long flags; > > spin_lock_irqsave(&x->wait.lock, flags); > x->done++; > __wake_up_locked(&x->wait, TASK_NORMAL, 1); > spin_unlock_irqrestore(&x->wait.lock, flags); > } > > and > > static inline long __sched > do_wait_for_common(struct completion *x, > long (*action)(long), long timeout, int state) > ... > spin_unlock_irq(&x->wait.lock); > timeout = action(timeout); > spin_lock_irq(&x->wait.lock); > > > so the power can't really be cut until the killing CPU sees the lock > released either explicitly via the second cache flush in cpu_die() or > implicitly via hardware.
Correct - so the caller of wait_for_completion_timeout() needs to re-acquire the cache line after the complete() in order to return successfully - which means that the spin_unlock_irqrestore() on the dying CPU _must_ have become visible to other observers for the requesting CPU to proceed.
> Maybe we can do the same thing here by using a > spinlock for synchronization between the IPI handler and the dying CPU? > So lock/unlock around the IPI sending from the dying CPU and then do a > lock/unlock on the killing CPU before continuing.
It would be nice, but it means exporting irq_controller_lock from irq_gic.c. It's doable, but it's really not nice - it creates a layering issue, buy making arch/arm/kernel/smp.c depend on symbols exported from drivers/irqchip/irq-gic.c.
> > Also, we _do_ need the second cache flush in place to ensure that the > > unlock is seen to other CPUs. > > > > We could work around that by taking and releasing the lock in the IPI > > processing function... but this is starting to look less attractive > > as the lock is private to irq-gic.c. > > With Daniel Thompson's NMI fiq patches at least the lock would almost > always be gone, except for the bL switcher users. Another solution might > be to put a hotplug lock around the bL switcher code and then skip > taking the lock in gic_raise_softirq() if the IPI is our special hotplug > one. Conditional locking is pretty ugly though, so perhaps this isn't > such a great idea.
I haven't even thought about the implications of that yet. :) We need to fix the already existing in-kernel code before we consider not-yet-merged code.
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