----- Original Message ----- > From: "Pranith Kumar" <[email protected]> > To: "Mathieu Desnoyers" <[email protected]> > Cc: "Huang Ying" <[email protected]>, "LKML" > <[email protected]>, "Paul McKenney" > <[email protected]>, "David Howells" <[email protected]> > Sent: Thursday, February 5, 2015 10:44:07 PM > Subject: Re: [PATCH] llist: Fix missing memory barrier > > Hi Mathieu, > > On Thu, Feb 5, 2015 at 10:06 PM, Mathieu Desnoyers > <[email protected]> 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?
Since llist.h has been introduced prior to 3.18, I'm wondering if it would be worthwhile to submit 2 patches for the purpose of backporting to stable branches: 1) Fix introducing smp_read_barrier_depends() (for master and stable branches) 2) Move master from smp_read_barrier_depends() to lockless_dereference(), Thoughts ? Thanks! Mathieu > > Thanks! > > [1] http://lkml.iu.edu/hypermail/linux/kernel/1410.3/04561.html > > > > > Signed-off-by: Mathieu Desnoyers <[email protected]> > > CC: Huang Ying <[email protected]> > > CC: Paul McKenney <[email protected]> > > CC: David Howells <[email protected]> > > --- > > 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 > > > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to [email protected] > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > > > > -- > Pranith > -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
