On Thu, Aug 27, 2020 at 06:12:43PM +0200, Peter Zijlstra wrote: > > > Cc: came...@moodycamel.com > Cc: o...@redhat.com > Cc: w...@kernel.org > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > --- > include/linux/freelist.h | 129 > +++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 129 insertions(+) > > --- /dev/null > +++ b/include/linux/freelist.h > @@ -0,0 +1,129 @@ > +// SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause > +#ifndef FREELIST_H > +#define FREELIST_H > + > +#include <linux/atomic.h> > + > +/* > + * Copyright: came...@moodycamel.com > + * > + * A simple CAS-based lock-free free list. Not the fastest thing in the world > + * under heavy contention, but simple and correct (assuming nodes are never > + * freed until after the free list is destroyed), and fairly speedy under low > + * contention. > + * > + * Adapted from: > https://moodycamel.com/blog/2014/solving-the-aba-problem-for-lock-free-free-lists > + */ > + > +struct freelist_node { > + atomic_t refs; > + struct freelist_node *next; > +}; > + > +struct freelist_head { > + struct freelist_node *head; > +}; > + > +#define REFS_ON_FREELIST 0x80000000 > +#define REFS_MASK 0x7FFFFFFF > + > +static inline void __freelist_add(struct freelist_node *node, struct > freelist_head *list) > +{ > + /* > + * Since the refcount is zero, and nobody can increase it once it's > + * zero (except us, and we run only one copy of this method per node at > + * a time, i.e. the single thread case), then we know we can safely > + * change the next pointer of the node; however, once the refcount is > + * back above zero, then other threads could increase it (happens under > + * heavy contention, when the refcount goes to zero in between a load > + * and a refcount increment of a node in try_get, then back up to > + * something non-zero, then the refcount increment is done by the other > + * thread) -- so if the CAS to add the node to the actual list fails, > + * decrese the refcount and leave the add operation to the next thread > + * who puts the refcount back to zero (which could be us, hence the > + * loop). > + */ > + struct freelist_node *head = READ_ONCE(list->head); > + > + for (;;) { > + WRITE_ONCE(node->next, head); > + atomic_set_release(&node->refs, 1); > + > + if (!try_cmpxchg_release(&list->head, &head, node)) { > + /* > + * Hmm, the add failed, but we can only try again when > + * the refcount goes back to zero. > + */ > + if (atomic_fetch_add_release(REFS_ON_FREELIST - 1, > &node->refs) == 1) > + continue; > + } > + return; > + } > +} > + > +static inline void freelist_add(struct freelist_node *node, struct > freelist_head *list) > +{ > + /* > + * We know that the should-be-on-freelist bit is 0 at this point, so > + * it's safe to set it using a fetch_add. > + */ > + if (!atomic_fetch_add_release(REFS_ON_FREELIST, &node->refs)) { > + /* > + * Oh look! We were the last ones referencing this node, and we > + * know we want to add it to the free list, so let's do it! > + */ > + __freelist_add(node, list); > + } > +} > + > +static inline struct freelist_node *freelist_try_get(struct freelist_head > *list) > +{ > + struct freelist_node *prev, *next, *head = > smp_load_acquire(&list->head); > + unsigned int refs; > + > + while (head) { > + prev = head; > + refs = atomic_read(&head->refs); > + if ((refs & REFS_MASK) == 0 || > + !atomic_try_cmpxchg_acquire(&head->refs, &refs, refs+1)) { > + head = smp_load_acquire(&list->head); > + continue; > + } > + > + /* > + * Good, reference count has been incremented (it wasn't at > + * zero), which means we can read the next and not worry about > + * it changing between now and the time we do the CAS. > + */ > + next = READ_ONCE(head->next); > + if (try_cmpxchg_acquire(&list->head, &head, next)) {
So if try_cmpxchg_acquire() fails, we don't have ACQUIRE semantics on read of the new list->head, right? Then probably a smp_mb__after_atomic() is needed in that case? Regards, Boqun > + /* > + * Yay, got the node. This means it was on the list, > + * which means should-be-on-freelist must be false no > + * matter the refcount (because nobody else knows it's > + * been taken off yet, it can't have been put back on). > + */ > + WARN_ON_ONCE(atomic_read(&head->refs) & > REFS_ON_FREELIST); > + > + /* > + * Decrease refcount twice, once for our ref, and once > + * for the list's ref. > + */ > + atomic_fetch_add(-2, &head->refs); > + > + return head; > + } > + > + /* > + * OK, the head must have changed on us, but we still need to > decrement > + * the refcount we increased. > + */ > + refs = atomic_fetch_add(-1, &prev->refs); > + if (refs == REFS_ON_FREELIST + 1) > + __freelist_add(prev, list); > + } > + > + return NULL; > +} > + > +#endif /* FREELIST_H */ > >
