Messages in this thread Patch in this message | | | Subject | Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro | From | Eric Dumazet <> | Date | Mon, 27 May 2013 17:12:10 -0700 |
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On Mon, 2013-05-27 at 21:55 +0400, Roman Gushchin wrote: > Hi, Paul! > > > On 25.05.2013 15:37, Paul E. McKenney wrote: > >> Again, I believe that your retry logic needs to extend back into the > >> calling function for your some_func() example above. > > And what do you think about the following approach (diff below)? > > It seems to me, it's enough clear (especially with good accompanying comments) > and produces a good binary code (without significant overhead). > Also, we will remove a hidden reef in using rcu-protected (h)list traverses with restarts. >
> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h > index 2ae1371..4af5ee5 100644 > --- a/include/linux/rculist_nulls.h > +++ b/include/linux/rculist_nulls.h > @@ -107,7 +107,8 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n, > * > */ > #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member) \ > - for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ > + for (ACCESS_ONCE(*(head)), \ > + pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ > (!is_a_nulls(pos)) && \ > ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \ > pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))
It looks like this still relies on gcc being friendly here.
I repeat again : @head here is a constant.
Macro already uses ACCESS_ONCE(), we only have to instruct gcc that caching the value is forbidden if we restart the loop (aka "goto begin;" see Documentation/RCU/rculist_nulls.txt line 146)
Adding a barrier() is probably what we want.
I cooked followed patch and it fixes the problem.
diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h index 2ae1371..4dc51b2 100644 --- a/include/linux/rculist_nulls.h +++ b/include/linux/rculist_nulls.h @@ -105,8 +105,12 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n, * @head: the head for your list. * @member: the name of the hlist_nulls_node within the struct. * + * The barrier() is needed to make sure compiler doesn't cache first element, + * as this loop can be restarted. + * (cf Documentation/RCU/rculist_nulls.txt around line 146) */ #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member) \ + barrier(); \ for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ (!is_a_nulls(pos)) && \ ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
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