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SubjectRe: [PATCH] llist: Fix missing memory barrier
Hi Mathieu,

On Thu, Feb 5, 2015 at 10:06 PM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
> A smp_read_barrier_depends() appears to be missing in llist_del_first().
> It should only matter for Alpha in practice. Adding it after the check
> of entry against NULL allows skipping the barrier in a common case.

We recently decided on using lockless_dereference() instead of
hard-coding smp_read_barrier_depends()[1]. The advantage is that
lockless_dereference() clearly shows what loads are being ordered.
Could you resend the patch using that API?

Thanks!

[1] http://lkml.iu.edu/hypermail/linux/kernel/1410.3/04561.html

>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: Huang Ying <ying.huang@intel.com>
> CC: Paul McKenney <paulmck@linux.vnet.ibm.com>
> CC: David Howells <dhowells@redhat.com>
> ---
> lib/llist.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/lib/llist.c b/lib/llist.c
> index f76196d..72861f3 100644
> --- a/lib/llist.c
> +++ b/lib/llist.c
> @@ -26,6 +26,7 @@
> #include <linux/export.h>
> #include <linux/interrupt.h>
> #include <linux/llist.h>
> +#include <asm/barrier.h>
>
>
> /**
> @@ -72,6 +73,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
> if (entry == NULL)
> return NULL;
> old_entry = entry;
> + /*
> + * Load entry before entry->next. Matches the implicit
> + * memory barrier before the cmpxchg in llist_add_batch(),
> + * which ensures entry->next is stored before entry.
> + */
> + smp_read_barrier_depends();
> next = entry->next;
> entry = cmpxchg(&head->first, old_entry, next);
> if (entry == old_entry)
> --
> 2.1.4
>
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--
Pranith


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