null_blk: remove hardcoded alloc_cmd() parameter

Only caller of alloc_cmd() is null_submit_bio() unconditionally sets
second parameter to true and that is statically hard-coded in null_blk.
There is no point in having statically hardcoded function parameter.

Remove the unnecessary parameter can_wait and adjust the code so it
can retain existing behavior of waiting when we don't get valid
nullb_cmd from __alloc_cmd() in alloc_cmd().

The restructured code avoids multiple return statements, multiple
calls to __alloc_cmd() and resulting a fast path call to
prepare_to_wait() due to removal of first alloc_cmd() call.

Follow the pattern that we have in bio_alloc() to set the structure
members in the structure allocation function in alloc_cmd() and pass
bio to initialize newly allocated cmd->bio member.

Follow the pattern in copy_to_nullb() to use result of one function call
(null_cache_active()) to be used as a parameter to another function call
(null_insert_page()), use result of alloc_cmd() as a first parameter to
the null_handle_cmd() in null_submit_bio() function. This allow us to
remove the local variable cmd on stack in null_submit_bio() that is in
fast path.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20220216172945.31124-2-kch@nvidia.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 90b6bd2..29e1837 100644 (file)
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -720,26 +720,25 @@ static struct nullb_cmd *__alloc_cmd(struct nullb_queue *nq)
        return NULL;
 }
 
-static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq, int can_wait)
+static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq, struct bio *bio)
 {
        struct nullb_cmd *cmd;
        DEFINE_WAIT(wait);
 
-       cmd = __alloc_cmd(nq);
-       if (cmd || !can_wait)
-               return cmd;
-
        do {
-               prepare_to_wait(&nq->wait, &wait, TASK_UNINTERRUPTIBLE);
+               /*
+                * This avoids multiple return statements, multiple calls to
+                * __alloc_cmd() and a fast path call to prepare_to_wait().
+                */
                cmd = __alloc_cmd(nq);
-               if (cmd)
-                       break;
-
+               if (cmd) {
+                       cmd->bio = bio;
+                       return cmd;
+               }
+               prepare_to_wait(&nq->wait, &wait, TASK_UNINTERRUPTIBLE);
                io_schedule();
+               finish_wait(&nq->wait, &wait);
        } while (1);
-
-       finish_wait(&nq->wait, &wait);
-       return cmd;
 }
 
 static void end_cmd(struct nullb_cmd *cmd)
@@ -1477,12 +1476,8 @@ static void null_submit_bio(struct bio *bio)
        sector_t nr_sectors = bio_sectors(bio);
        struct nullb *nullb = bio->bi_bdev->bd_disk->private_data;
        struct nullb_queue *nq = nullb_to_queue(nullb);
-       struct nullb_cmd *cmd;
-
-       cmd = alloc_cmd(nq, 1);
-       cmd->bio = bio;
 
-       null_handle_cmd(cmd, sector, nr_sectors, bio_op(bio));
+       null_handle_cmd(alloc_cmd(nq, bio), sector, nr_sectors, bio_op(bio));
 }
 
 static bool should_timeout_request(struct request *rq)