linux-kernel - Re: [PATCH RFC] hwspinlock: Don't take software spinlock before hwspinlock

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Date:	Mon, 11 May 2015 08:46:08 -0600
From:	Lina Iyer <lina.iyer@...aro.org>
To:	Ohad Ben-Cohen <ohad@...ery.com>
Cc:	"Anna, Suman" <s-anna@...com>,
	Bjorn Andersson <Bjorn.Andersson@...ymobile.com>,
	Andy Gross <agross@...eaurora.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kumar Gala <galak@...eaurora.org>,
	Jeffrey Hugo <jhugo@...eaurora.org>
Subject: Re: [PATCH RFC] hwspinlock: Don't take software spinlock before
 hwspinlock

On Sat, May 09 2015 at 03:25 -0600, Ohad Ben-Cohen wrote:
>Hi Lina,
>
>On Fri, May 1, 2015 at 8:07 PM, Lina Iyer <lina.iyer@...aro.org> wrote:
>> Some uses of the hwspinlock could be that one entity acquires the lock
>> and the other entity releases the lock. This allows for a serialized
>> traversal path from the locking entity to the other.
>>
>> For example, the cpuidle entry from Linux to the firmware to power down
>> the core, can be serialized across the context switch by locking the
>> hwspinlock in Linux and releasing it in the firmware.
>>
>> Do not force the caller of __hwspin_trylock() to acquire a kernel
>> spinlock before acquiring the hwspinlock.
>
>Let's discuss whether we really want to expose this functionality
>under the same hwspinlock API or not.
>
>In this new mode, unlike previously, users will now be able to sleep
>after taking the lock, and others trying to take the lock might poll
>the hardware for a long period of time without the ability to sleep
>while waiting for the lock. It almost sounds like you were looking for
>some hwmutex functionality.
>
>What do you think about this?

I agree, that it opens up a possiblity that user may sleep after holding
a hw spinlock.  But really, why should it prevents us from using it as a
hw mutex, if the need is legitimate?

We could make a check that the caller with NO_LOCK option calls only
with irq disabled, if thats required.

Thanks for the review.

-- Lina
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