linux-kernel - Re: [GIT PULL rcu/next] RCU commits for 3.1

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Message-ID: <1320676871.2361.21.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Mon, 07 Nov 2011 15:41:11 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Li Zefan <lizf@...fujitsu.com>, paulmck@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>, shaohua.li@...el.com,
	ak@...ux.intel.com, mhocko@...e.cz, alex.shi@...el.com,
	efault@....de, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [GIT PULL rcu/next] RCU commits for 3.1

Le lundi 07 novembre 2011 à 14:24 +0000, Stephane Eranian a écrit :
> Hi,
> 
> Some second thoughts on this.
> 
> We get the warning because:
> 
> #define task_subsys_state_check(task, subsys_id, __c)                   \
>         rcu_dereference_check(task->cgroups->subsys[subsys_id],         \
>                               lockdep_is_held(&task->alloc_lock) ||     \
>                               cgroup_lock_is_held() || (__c))
> 
> 
> In other words, we need the alloc_lock held.
> 
> What I don't quite understand in your patch in the connection
> between rcu and this particular task lock. Unless holding
> the rcu read lock implies you necessarily hold the alloc_lock.
> 
> Can you explain?

#define rcu_dereference_check(p, c) \
	__rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)

So if you hold rcu_read_lock(), (c) is not checked/asserted.



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