On Thu, Dec 8, 2016 at 6:12 PM, Huang, Ying <[email protected]> wrote: > Joel Fernandes <[email protected]> writes: > >> On Thu, Dec 8, 2016 at 4:42 PM, Joel Fernandes <[email protected]> wrote: >>> On Thu, Dec 8, 2016 at 4:35 PM, Huang, Ying <[email protected]> wrote: >>>> Joel Fernandes <[email protected]> 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 <[email protected]> >>>>> Cc: Ingo Molnar <[email protected]> >>>>> Cc: Will Deacon <[email protected]> >>>>> Cc: Paul McKenney <[email protected]> >>>>> Signed-off-by: Joel Fernandes <[email protected]> >>>>> --- >>>>> 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.
Ok, now I get it - so basically the table describes one producer/consumer vs another producer/consumer, in other words you are just describing contention between any 2 operations. Thanks for clarifying. I will respin the comments to explain this a bit better if that's Ok with you. Regards, Joel
