Message-ID: <54D95DB8.9010308@codeaurora.org>
Date: Mon, 09 Feb 2015 17:24:08 -0800
From: Stephen Boyd <sboyd@codeaurora.org>
User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0
MIME-Version: 1.0
To: Russell King - ARM Linux <linux@arm.linux.org.uk>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
CC: Krzysztof Kozlowski <k.kozlowski@samsung.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>, Mark Rutland <mark.rutland@arm.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v2] ARM: Don't use complete() during __cpu_die
References: <1423131270-24047-1-git-send-email-k.kozlowski@samsung.com> <20150205105035.GL8656@n2100.arm.linux.org.uk> <20150205142918.GA10634@linux.vnet.ibm.com> <20150205161100.GQ8656@n2100.arm.linux.org.uk>
In-Reply-To: <20150205161100.GQ8656@n2100.arm.linux.org.uk>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 3033
Lines: 72

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.

>
> 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.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/