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Date: Wed, 11 Apr 2012 17:30:44 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Michel Machado <michel@...irati.com.br> 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 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 > 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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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