Messages in this thread Patch in this message | | | From | Joel Fernandes <> | Subject | [RFC] llist: Fix code comments about llist_del_first locking | Date | Thu, 8 Dec 2016 13:54:40 -0800 |
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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 | - * del_first | | L | L * del_all | | | - * -- 2.8.0.rc3.226.g39d4020
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