linux-kernel - Re: Sleeping in RCU list traversal

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Date:	Mon, 08 Oct 2007 06:37:20 +0800
From:	Jun WANG <junwang1234@...il.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Sleeping in RCU list traversal


> Hello.
> 
> Thank you for pointing out.
> 
> Jun WANG wrote:
> > >rcu_read_lock();
> > >list_for_each_rcu(p, ...) {
> > >   ptr = list_entry(p, struct ..., list);
> > >   /* Grab a reference to "ptr". */
> > >   rcu_read_unlock();
> > >   my_task_that_may_sleep(ptr);
> > >   rcu_read_lock();
> > >   /* Drop a reference to "ptr". */
> > > }
> > > rcu_read_unlock();
>>>Regarding my case, memory region pointed by "ptr" never be removed.
>>>Do I need to grab a reference to "ptr" ?
> > 
> > In Document/RCU/whatisRCU.txt
> > Note that the value returned by rcu_dereference() is valid
> > 	only within the enclosing RCU read-side critical section.
> Excuse me, but I think "p" is used only between rcu_read_lock() and rcu_read_unlock().
> Is it illegal to use "ptr" after rcu_read_unlock() if "ptr" is obtained before rcu_read_unlock() ?
> 
> Regards.
> 
I'm sorry,I think I got your idea, if you do not need ptr in
my_task_that_may_sleep(), why you need to grab a reference to "ptr". If
your my_task_that_may_sleep() needs ptr, and according to the 
>"memory region pointed by "ptr" never be removed." you say,
it is ok to use "ptr" after rcu_read_ulock(). The basic idea behind RCU
is to split updates into "removal" and "reclamation" phases. If you
memory region pointed by "ptr" will not "reclamation" in sleep, it is ok

Regards.

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