Messages in this thread | | | From | "Huang\, Ying" <> | Subject | Re: [RFC] llist: Fix code comments about llist_del_first locking | Date | Fri, 09 Dec 2016 10:12:39 +0800 |
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Joel Fernandes <joelaf@google.com> writes:
> On Thu, Dec 8, 2016 at 4:42 PM, Joel Fernandes <joelaf@google.com> wrote: >> On Thu, Dec 8, 2016 at 4:35 PM, Huang, Ying <ying.huang@intel.com> wrote: >>> Joel Fernandes <joelaf@google.com> writes: >>> >>>> Usage llist_del_first needs lock protection, however the table in the >>>> comments of llist.h show a '-'. Correct this, and also add better >>>> comments on top. >>>> >>>> Cc: Huang Ying <ying.huang@intel.com> >>>> Cc: Ingo Molnar <mingo@kernel.org> >>>> Cc: Will Deacon <will.deacon@arm.com> >>>> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> >>>> Signed-off-by: Joel Fernandes <joelaf@google.com> >>>> --- >>>> include/linux/llist.h | 19 ++++++++++--------- >>>> 1 file changed, 10 insertions(+), 9 deletions(-) >>>> >>>> diff --git a/include/linux/llist.h b/include/linux/llist.h >>>> index fd4ca0b..15e4949 100644 >>>> --- a/include/linux/llist.h >>>> +++ b/include/linux/llist.h >>>> @@ -3,14 +3,15 @@ >>>> /* >>>> * Lock-less NULL terminated single linked list >>>> * >>>> - * If there are multiple producers and multiple consumers, llist_add >>>> - * can be used in producers and llist_del_all can be used in >>>> - * consumers. They can work simultaneously without lock. But >>>> - * llist_del_first can not be used here. Because llist_del_first >>>> - * depends on list->first->next does not changed if list->first is not >>>> - * changed during its operation, but llist_del_first, llist_add, >>>> - * llist_add (or llist_del_all, llist_add, llist_add) sequence in >>>> - * another consumer may violate that. >>>> + * If there are multiple producers and multiple consumers, llist_add can be >>>> + * used in producers and llist_del_all can be used in consumers. They can work >>>> + * simultaneously without lock. But llist_del_first will need to use a lock >>>> + * with any other operation (ABA problem). This is because llist_del_first >>>> + * depends on list->first->next not changing but there's no way to be sure >>>> + * about that and the cmpxchg in llist_del_first may succeed if list->first is >>>> + * the same after concurrent operations. For example, a llist_del_first, >>>> + * llist_add, llist_add (or llist_del_all, llist_add, llist_add) sequence in >>>> + * another consumer may cause violations. >>>> * >>>> * If there are multiple producers and one consumer, llist_add can be >>>> * used in producers and llist_del_all or llist_del_first can be used >>>> @@ -19,7 +20,7 @@ >>>> * This can be summarized as follow: >>>> * >>>> * | add | del_first | del_all >>>> - * add | - | - | - >>>> + * add | - | L | - >>> >>> If there are only one consumer which only calls llist_del_first(), lock >>> is unnecessary. So '-' is shown here originally. But if there are >>> multiple consumers which call llist_del_first() or llist_del_all(), lock >>> is needed. >> >> I think this needs to be made more clear in the table. The table >> doesn't clear say whether it describes the preceding paragraph >> (multiple producers and one consumer), or if it describes the multiple >> producers and one consumer case. So either we should have 2 tables, or > > Sorry, I meant "or if it describes the multiple producer and multiple > consumer case".
I tried to describe both cases in the original table.
* | add | del_first | del_all * add | - | - | - * del_first | | L | L * del_all | | | -
The 'L' for "del_first * del_first" means multiple consumers uses llist_del_first() need lock. And the 'L' for 'del_first * del_all' means multiple consumers uses llist_del_first() and llist_del_all() need lock.
Best Regards, Huang, Ying
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