[PATCH] mtip32xx: avoid using semaphores

[PATCH] mtip32xx: avoid using semaphores

[Arnd Bergmann]

The "cmd_slot_unal" semaphore is never used in a blocking way
but only as an atomic counter. Change the code to using
atomic_dec_if_positive() as a better API.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/block/mtip32xx/mtip32xx.c | 6 +++---
 drivers/block/mtip32xx/mtip32xx.h | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/block/mtip32xx/mtip32xx.c b/drivers/block/mtip32xx/mtip32xx.c
index 2b0ac9d01e51..d7dc307d0cc8 100644
--- a/drivers/block/mtip32xx/mtip32xx.c
+++ b/drivers/block/mtip32xx/mtip32xx.c
@@ -2680,7 +2680,7 @@ static void mtip_softirq_done_fn(struct request *rq)
 							cmd->direction);
 
 	if (unlikely(cmd->unaligned))
-		up(&dd->port->cmd_slot_unal);
+		atomic_inc(&dd->port->cmd_slot_unal);
 
 	blk_mq_end_request(rq, cmd->status);
 }
@@ -2990,7 +2990,7 @@ static int mtip_hw_init(struct driver_data *dd)
 	else
 		dd->unal_qdepth = 0;
 
-	sema_init(&dd->port->cmd_slot_unal, dd->unal_qdepth);
+	atomic_set(&dd->port->cmd_slot_unal, dd->unal_qdepth);
 
 	/* Spinlock to prevent concurrent issue */
 	for (i = 0; i < MTIP_MAX_SLOT_GROUPS; i++)
@@ -3533,7 +3533,7 @@ static bool mtip_check_unal_depth(struct blk_mq_hw_ctx *hctx,
 			cmd->unaligned = 1;
 	}
 
-	if (cmd->unaligned && down_trylock(&dd->port->cmd_slot_unal))
+	if (cmd->unaligned && atomic_dec_if_positive(&dd->port->cmd_slot_unal) >= 0)
 		return true;
 
 	return false;
diff --git a/drivers/block/mtip32xx/mtip32xx.h b/drivers/block/mtip32xx/mtip32xx.h
index c33f8c3d9fb4..abce25f27f57 100644
--- a/drivers/block/mtip32xx/mtip32xx.h
+++ b/drivers/block/mtip32xx/mtip32xx.h
@@ -433,8 +433,8 @@ struct mtip_port {
 	 */
 	unsigned long ic_pause_timer;
 
-	/* Semaphore to control queue depth of unaligned IOs */
-	struct semaphore cmd_slot_unal;
+	/* Counter to control queue depth of unaligned IOs */
+	atomic_t cmd_slot_unal;
 
 	/* Spinlock for working around command-issue bug. */
 	spinlock_t cmd_issue_lock[MTIP_MAX_SLOT_GROUPS];
-- 
2.20.0

Re: [PATCH] mtip32xx: avoid using semaphores

[Jens Axboe]

On 12/10/18 2:34 PM, Arnd Bergmann wrote:
> The "cmd_slot_unal" semaphore is never used in a blocking way
> but only as an atomic counter. Change the code to using
> atomic_dec_if_positive() as a better API.

Looks good to me. We should also convert the return to blk-mq
when we hit failure here to BLK_STS_DEV_RESOURCE, but that's
a separate patch.

-- 
Jens Axboe