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Date: Mon, 9 Feb 2015 17:37:23 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Stephen Boyd <sboyd@...eaurora.org> Cc: Russell King - ARM Linux <linux@....linux.org.uk>, Krzysztof Kozlowski <k.kozlowski@...sung.com>, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, Mark Rutland <mark.rutland@....com>, Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>, Marek Szyprowski <m.szyprowski@...sung.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com> Subject: Re: [PATCH v2] ARM: Don't use complete() during __cpu_die 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. 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 if we didn't have to do anything at all though so > perhaps we can make it a nop on configs where there isn't a big little > switcher. Yeah it's some ugly coupling between these two pieces of code, > but I'm not sure how we can do better. The default ugly-but-known-to-work approach is to set a variable in the dying CPU that the surviving CPU periodically polls. If all else fails and all that. > > 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. Which hotplug lock are you suggesting? We cannot use sleeplocks, because releasing them can go through the scheduler, which is not legal at this point. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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