Messages in this thread | | | Date | Tue, 2 Apr 2013 09:05:42 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] wfcqueue: add function for unsynchronized prepend |
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* Eric Wong (normalperson@yhbt.net) wrote: > In some situations, it is necessary to prepend a node to a queue. > For epoll, this is necessary for two rare conditions: > > * when the user triggers -EFAULT > * when reinjecting elements from the ovflist (implemented as a stack)
This approach makes sense.
In terms of API naming, I wonder if "prepend" is the good counterpart for "enqueue". Maybe "enqueue_first" or "enqueue_head" could be better suited ?
Currently, we have an "append" function used internally, but it's not exposed by the API.
Just for fun, I tried making your "prepend" wait-free (thus not requiring a mutex), but it's really not obvious, because of its impact on splice operation and dequeue-last-node operation.
Thoughts ?
Thanks,
Mathieu
> > Signed-off-by: Eric Wong <normalperson@yhbt.net> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > --- > This is on top of my other patch to implement __wfcq_enqueue > > include/linux/wfcqueue.h | 35 +++++++++++++++++++++++++++-------- > 1 file changed, 27 insertions(+), 8 deletions(-) > > diff --git a/include/linux/wfcqueue.h b/include/linux/wfcqueue.h > index a452ab9..4cb8f22 100644 > --- a/include/linux/wfcqueue.h > +++ b/include/linux/wfcqueue.h > @@ -56,15 +56,17 @@ > * [5] __wfcq_first > * [6] __wfcq_next > * [7] __wfcq_enqueue > + * [8] __wfcq_prepend > * > - * [1] [2] [3] [4] [5] [6] [7] > - * [1] - - - - - - X > - * [2] - - - - - - X > - * [3] - - X X X X X > - * [4] - - X - X X X > - * [5] - - X X - - X > - * [6] - - X X - - X > - * [7] X X X X X X X > + * [1] [2] [3] [4] [5] [6] [7] [8] > + * [1] - - - - - - X X > + * [2] - - - - - - X X > + * [3] - - X X X X X X > + * [4] - - X - X X X X > + * [5] - - X X - - X X > + * [6] - - X X - - X X > + * [7] X X X X X X X X > + * [8] X X X X X X X X > * > * Besides locking, mutual exclusion of dequeue, splice and iteration > * can be ensured by performing all of those operations from a single > @@ -441,6 +443,23 @@ static inline enum wfcq_ret __wfcq_splice( > } > > /* > + * __wfcq_prepend: prepend a node into a queue, requiring mutual exclusion. > + * > + * No memory barriers are issued. Mutual exclusion is the responsibility > + * of the caller. > + */ > +static inline void __wfcq_prepend(struct wfcq_head *head, > + struct wfcq_tail *tail, struct wfcq_node *node) > +{ > + node->next = head->node.next; > + head->node.next = node; > + > + /* if the queue was empty before, it is no longer empty now */ > + if (tail->p == &head->node) > + tail->p = node; > +} > + > +/* > * __wfcq_for_each: Iterate over all nodes in a queue, > * without dequeuing them. > * @head: head of the queue (struct wfcq_head pointer). > -- > Eric Wong >
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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