linux-kernel - Re: [PATCH 1/1] rculist: Replaced list_first_entry_rcu() with list_first_or_null

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Date:	Wed, 11 Apr 2012 21:16:35 -0400
From:	Michel Machado <michel@...irati.com.br>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Dipankar Sarma <dipankar@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] rculist: Replaced list_first_entry_rcu() with
 list_first_or_null_rcu()

On Wed, 2012-04-11 at 17:30 -0700, Paul E. McKenney wrote:
> On Tue, Apr 10, 2012 at 02:07:40PM -0400, Michel Machado wrote:
> > Replaced list_first_entry_rcu() with list_first_or_null_rcu() because
> > list_first_entry_rcu() is not safe as one can find in the comment that
> > this patch also adds.
> > 
> > This patch incorporated Paul's suggestions to the previous version of
> > this patch available here:
> > 
> > https://lkml.org/lkml/2012/4/2/536
> > 
> > This patch cannot break any upstream code because list_first_entry_rcu
> > is not being used anywhere in the kernel (tested with grep(1)), and
> > external code that uses it is probably broken already.
> 
> Thank you, Michel, I have queued this.
> 
> However, in the future, could you please configure your email client to
> avoid breaking lines and could you please also run scripts/checkpatch.pl
> on future patches?  Applying your patch took some hand-editing to merge
> the broken lines and to remove trailing spaces on lines.
> 
> 							Thanx, Paul
> 

Sorry for the extra work, I'll pay more attention next time.

[ ]'s
Michel Machado

> > Signed-off-by: Michel Machado <michel@...irati.com.br>
> > CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > CC: Dipankar Sarma <dipankar@...ibm.com>
> > ---
> > 
> > diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> > index d079290..ef5da30 100644
> > --- a/include/linux/rculist.h
> > +++ b/include/linux/rculist.h
> > @@ -228,18 +228,43 @@ static inline void list_splice_init_rcu(struct
> > list_head *list,
> >  	})
> > 
> >  /**
> > - * list_first_entry_rcu - get the first element from a list
> > + * Where are list_empty_rcu() and list_first_entry_rcu()?
> > + *
> > + * Implementing those functions following their counterparts
> > list_empty() and
> > + * list_first_entry() is not advisable because they lead to subtle race
> > + * conditions as the following snippet shows:
> > + * 
> > + * if (!list_empty_rcu(mylist)) {
> > + * 	struct foo *bar = list_first_entry_rcu(mylist, struct foo,
> > list_member);
> > + * 	do_something(bar);
> > + * }
> > + *
> > + * The list may not be empty when list_empty_rcu checks it, but it may
> > be when
> > + * list_first_entry_rcu rereads the ->next pointer.
> > + *
> > + * Rereading the ->next pointer is not a problem for list_empty() and
> > + * list_first_entry() because they would be protected by a lock that
> > blocks
> > + * writers.
> > + *
> > + * See list_first_or_null_rcu for an alternative.
> > + */
> > +
> > +/**
> > + * list_first_or_null_rcu - get the first element from a list
> >   * @ptr:        the list head to take the element from.
> >   * @type:       the type of the struct this is embedded in.
> >   * @member:     the name of the list_struct within the struct.
> >   *
> > - * Note, that list is expected to be not empty.
> > + * Note that if the list is empty, it returns NULL.
> >   *
> >   * This primitive may safely run concurrently with the _rcu
> > list-mutation
> >   * primitives such as list_add_rcu() as long as it's guarded by
> > rcu_read_lock().
> >   */
> > -#define list_first_entry_rcu(ptr, type, member) \
> > -	list_entry_rcu((ptr)->next, type, member)
> > +#define list_first_or_null_rcu(ptr, type, member) \
> > +	({struct list_head *__ptr = (ptr); \
> > +	  struct list_head __rcu *__next = list_next_rcu(__ptr); \
> > +	  likely(__ptr != __next) ? container_of(__next, type, member) : NULL;
> > \
> > +	})
> > 
> >  /**
> >   * list_for_each_entry_rcu	-	iterate over rcu list of given type
> > 
> > --
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> > Please read the FAQ at  http://www.tux.org/lkml/
> > 
> 

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