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

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Date:	Wed, 6 Mar 2013 15:00:52 +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 14:22, Tejun Heo wrote:
> Hello, Li.
> 
> On Wed, Mar 06, 2013 at 11:28:01AM +0800, Li Zefan wrote:
>> cgroup_event_wake() is called with hardirq-safe wqh->lock held, so
>> the nested cgrp->event_list_lock should also be hardirq-safe.
>>
>> Fortunately I don't think the deadlock can happen in real life.
>>
>> Lockdep never complained, maybe because it never found wqh->lock was
>> held in irq context?
> 
> 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.

> Locks which are grabbed with irq disabled aren't
> necessarily irq context locks as that doesn't lead to deadlocks.  They
> need to be actually grabbed from irq context.
> 
> Thanks.
> 

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