On Thu, Feb 07 2008, Linus Torvalds wrote:
>
>
> On Thu, 7 Feb 2008, Ingo Molnar wrote:
> > INIT_HLIST_NODE(&rq->hash);
> > RB_CLEAR_NODE(&rq->rb_node);
> > - rq->ioprio = 0;
> > - rq->buffer = NULL;
> > - rq->ref_count = 1;
> > - rq->q = q;
> > - rq->special = NULL;
> > - rq->data_len = 0;
> > - rq->data = NULL;
> > - rq->nr_phys_segments = 0;
> > - rq->sense = NULL;
> > - rq->end_io = NULL;
> > - rq->end_io_data = NULL;
> > - rq->completion_data = NULL;
> > - rq->next_rq = NULL;
> > + rq->completion_data = NULL;
> > + /* rq->elevator_private */
> > + /* rq->elevator_private2 */
> > + /* rq->rq_disk */
> > + /* rq->start_time */
> > + rq->nr_phys_segments = 0;
> > + /* rq->nr_hw_segments */
> > + rq->ioprio = 0;
> > + rq->special = NULL;
> > + rq->buffer = NULL;
> ...
>
> Can we please just stop doing these one-by-one assignments, and just do
> something like
>
> memset(rq, 0, sizeof(*rq));
> rq->q = q;
> rq->ref_count = 1;
> INIT_HLIST_NODE(&rq->hash);
> RB_CLEAR_NODE(&rq->rb_node);
>
> instead?
>
> The memset() is likely faster and smaller than one-by-one assignments
> anyway, even if the one-by-ones can avoid initializing some field or there
> ends up being a double initialization..
Looked into this approach and we can't currently do that here, since
some members of the request are being set from blk_alloc_request() and
then from the io scheduler attaching private data to it. So we have to
preserve ->cmd_flags and ->elevator_private and ->elevator_private2 at
least. Now rq_init() is also used for stored requests, so we cannot just
rely on clearing at allocation time.
So I'd prefer being a little conservative here. The below reorders
rq_init() a bit and clears some more members to be on the safer side,
adding comments to why we cannot memset and an associated comment in
blkdev.h.
diff --git a/block/blk-core.c b/block/blk-core.c
index 4afb39c..fba4ca7 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -102,27 +102,38 @@ struct backing_dev_info *blk_get_backing_dev_info(struct
block_device *bdev)
}
EXPORT_SYMBOL(blk_get_backing_dev_info);
+/*
+ * We can't just memset() the structure, since the allocation path
+ * already stored some information in the request.
+ */
void rq_init(struct request_queue *q, struct request *rq)
{
INIT_LIST_HEAD(&rq->queuelist);
INIT_LIST_HEAD(&rq->donelist);
-
- rq->errors = 0;
+ rq->q = q;
+ rq->sector = rq->hard_sector = (sector_t) -1;
+ rq->nr_sectors = rq->hard_nr_sectors = 0;
+ rq->current_nr_sectors = rq->hard_cur_sectors = 0;
rq->bio = rq->biotail = NULL;
INIT_HLIST_NODE(&rq->hash);
RB_CLEAR_NODE(&rq->rb_node);
+ rq->rq_disk = NULL;
+ rq->nr_phys_segments = 0;
+ rq->nr_hw_segments = 0;
rq->ioprio = 0;
+ rq->special = NULL;
rq->buffer = NULL;
+ rq->tag = -1;
+ rq->errors = 0;
rq->ref_count = 1;
- rq->q = q;
- rq->special = NULL;
+ rq->cmd_len = 0;
+ memset(rq->cmd, 0, sizeof(rq->cmd));
rq->data_len = 0;
+ rq->sense_len = 0;
rq->data = NULL;
- rq->nr_phys_segments = 0;
rq->sense = NULL;
rq->end_io = NULL;
rq->end_io_data = NULL;
- rq->completion_data = NULL;
rq->next_rq = NULL;
}
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 90392a9..e1888cc 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -137,7 +137,9 @@ enum rq_flag_bits {
#define BLK_MAX_CDB 16
/*
- * try to put the fields that are referenced together in the same cacheline
+ * try to put the fields that are referenced together in the same cacheline.
+ * if you modify this structure, be sure to check block/blk-core.c:rq_init()
+ * as well!
*/
struct request {
struct list_head queuelist;
--
Jens Axboe
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