linux-kernel - Re: [PATCH] cgroup: make cgrp->event_list

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Message-ID: <5136ED2F.8080400@huawei.com>
Date:	Wed, 6 Mar 2013 15:15:59 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	"Kirill A. Shutemov" <kirill@...temov.name>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH] cgroup: make cgrp->event_list_lock irqsafe

On 2013/3/6 15:02, Tejun Heo wrote:
> On Tue, Mar 5, 2013 at 11:00 PM, Li Zefan <lizefan@...wei.com> wrote:
>>> Why should wqh->lock be hard-irq-safe?  Is it actually grabbed from
>>> irq context?
>>
>> becase cgroup_event_wake() is a callback to a wait queue, and it's wake_up()
>> that acquires wqh->lock with irq disabled.
> 
> So, acquiring a lock with irq disabled doesn't make it a irq lock.
> Being grabbed *from* irq handler makes it a irq lock. Would the
> wake_up() happen from irq handler?
> 

wqh->lock is used through out fs/eventfd.c. I don't know if currently there's
any kernel user using eventfd APIs in an irq handler, but at least that should
be allowed.

wake_up() is also allowed to be called from irq handler?

"allowed" should be enough reason we forbid:

	spin_lock_irqsave(...)
	spin_lock(...)

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