lkml.org 
[lkml]   [2013]   [May]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro
On 21.05.2013 16:09, Paul E. McKenney wrote:
> On Tue, May 21, 2013 at 01:05:48PM +0400, Roman Gushchin wrote:
>> Hi, all!
>>
>> This is a fix for a problem described here:
>> https://lkml.org/lkml/2013/4/16/371 .
>> ---
>>
>> Some network functions (udp4_lib_lookup2(), for instance) use the
>> hlist_nulls_for_each_entry_rcu macro in a way that assumes restarting
>> of a loop. In this case, it is strictly necessary to reread the head->first
>> value from the memory before each scan.
>> Without additional hints, gcc caches this value in a register. In this case,
>> if a cached node is moved to another chain during the scan, we can loop
>> forever getting wrong nulls values and restarting the loop uninterruptedly.
>>
>> Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
>> Reported-by: Boris Zhmurov <zhmurov@yandex-team.ru>
>> ---
>> include/linux/rculist_nulls.h | 5 +++--
>> 1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
>> index 2ae1371..efd51bf 100644
>> --- a/include/linux/rculist_nulls.h
>> +++ b/include/linux/rculist_nulls.h
>> @@ -37,8 +37,9 @@ static inline void hlist_nulls_del_init_rcu(struct
>> hlist_nulls_node *n)
>> }
>> }
>>
>> -#define hlist_nulls_first_rcu(head) \
>> - (*((struct hlist_nulls_node __rcu __force **)&(head)->first))
>> +#define hlist_nulls_first_rcu(head) \
>> + (*((struct hlist_nulls_node __rcu __force **) \
>> + &((volatile typeof(*head) *)head)->first))
>
> Why not use ACCESS_ONCE() or (better) rcu_dereference_raw() here?

It will be nice, but will require to keep the old variant too (for using
in hlist_nulls_add_head_rcu() as in rcu_assign_pointer() argument). Do
you think, it's better?

Regards,
Roman



\
 
 \ /
  Last update: 2013-05-21 15:21    [W:0.109 / U:1.764 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site