Messages in this thread | | | From | Jann Horn <> | Date | Tue, 24 Mar 2020 17:20:45 +0100 | Subject | Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race() |
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On Tue, Mar 24, 2020 at 4:37 PM Will Deacon <will@kernel.org> wrote: > Some list predicates can be used locklessly even with the non-RCU list > implementations, since they effectively boil down to a test against > NULL. For example, checking whether or not a list is empty is safe even > in the presence of a concurrent, tearing write to the list head pointer. > Similarly, checking whether or not an hlist node has been hashed is safe > as well. > > Annotate these lockless list predicates with data_race() and READ_ONCE() > so that KCSAN and the compiler are aware of what's going on. The writer > side can then avoid having to use WRITE_ONCE() in the non-RCU > implementation. [...] > static inline int list_empty(const struct list_head *head) > { > - return READ_ONCE(head->next) == head; > + return data_race(READ_ONCE(head->next) == head); > } [...] > static inline int hlist_unhashed(const struct hlist_node *h) > { > - return !READ_ONCE(h->pprev); > + return data_race(!READ_ONCE(h->pprev)); > }
This is probably valid in practice for hlist_unhashed(), which compares with NULL, as long as the most significant byte of all kernel pointers is non-zero; but I think list_empty() could realistically return false positives in the presence of a concurrent tearing store? This could break the following code pattern:
/* optimistic lockless check */ if (!list_empty(&some_list)) { /* slowpath */ mutex_lock(&some_mutex); list_for_each(tmp, &some_list) { ... } mutex_unlock(&some_mutex); }
(I'm not sure whether patterns like this appear commonly though.)
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