linux-kernel - Re: [PATCH] RCU: Better explain the condition parameter of rcu_dereference

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Message-ID: <20100408043623.GB2887@linux.vnet.ibm.com>
Date:	Wed, 7 Apr 2010 21:36:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yong Zhang <yong.zhang@...driver.com>
Cc:	David Howells <dhowells@...hat.com>, eric.dumazet@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RCU: Better explain the condition parameter of
 rcu_dereference_check()

On Thu, Apr 08, 2010 at 10:50:50AM +0800, Yong Zhang wrote:
> On Wed, Apr 07, 2010 at 06:15:00PM +0100, David Howells wrote:
> > Better explain the condition parameter of rcu_dereference_check() that
> > describes the conditions under which the dereference is permitted to take
> > place.  This condition is only checked under lockdep proving.
> > 
> > Signed-off-by: David Howells <dhowells@...hat.com>
> > ---
> > 
> >  include/linux/rcupdate.h |   28 +++++++++++++++++++++++-----
> >  1 files changed, 23 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 872a98e..acf9958 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -195,12 +195,30 @@ static inline int rcu_read_lock_sched_held(void)
> >  
> >  /**
> >   * rcu_dereference_check - rcu_dereference with debug checking
> > + * @p: The pointer to read, prior to dereferencing
> > + * @c: The conditions under which the dereference will take place
> >   *
> > - * Do an rcu_dereference(), but check that the context is correct.
> > - * For example, rcu_dereference_check(gp, rcu_read_lock_held()) to
> > - * ensure that the rcu_dereference_check() executes within an RCU
> > - * read-side critical section.  It is also possible to check for
> > - * locks being held, for example, by using lockdep_is_held().
> > + * Do an rcu_dereference(), but check that the conditions under which the
> > + * dereference will take place are correct.  Typically the conditions indicate
> > + * the various locking conditions that should be held at that point.  The check
> > + * should return true if the conditions are satisfied.
> > + *
> > + * For example:
> > + *
> > + *	bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
> > + *					      lockdep_is_held(&foo->lock));
> > + *
> > + * could be used to indicate to lockdep that foo->bar may only be dereferenced
> > + * if either the RCU read lock is held, or that the lock required to replace
> > + * the bar struct at foo->bar is held.
> > + *
> > + * Note that the list of conditions may also include indications of when a lock
> > + * need not be held, for example during initialisation or destruction of the
> > + * target struct:
> > + *
> > + *	bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
> > + *					      lockdep_is_held(&foo->lock),
>                                                                         ^^
> 									|| ?

Good catch, fixed!

							Thanx, Paul

> Thanks,
> Yong
> 
> > + *					      atomic_read(&foo->usage) == 0);
> >   */
> >  #define rcu_dereference_check(p, c) \
> >  	({ \
> > 
> > --
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