linux-kernel - [PATCH 04/13] scsi: get rid of lock in __scsi_put

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Date:	Mon, 25 May 2009 09:30:50 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:	chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
	akpm@...ux-foundation.org, jack@...e.cz,
	yanmin_zhang@...ux.intel.com, Jens Axboe <jens.axboe@...cle.com>
Subject: [PATCH 04/13] scsi: get rid of lock in __scsi_put_command()

A memory barrier is enough to check for the list being empty
or not, then we only have to grab the lock if we stole the
reserved command (which is very unlikely).

Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
---
 drivers/scsi/scsi.c |   10 +++++-----
 1 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 6a993af..da33b7a 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -293,15 +293,15 @@ EXPORT_SYMBOL(scsi_get_command);
 void __scsi_put_command(struct Scsi_Host *shost, struct scsi_cmnd *cmd,
 			struct device *dev)
 {
-	unsigned long flags;
-
-	/* changing locks here, don't need to restore the irq state */
-	spin_lock_irqsave(&shost->free_list_lock, flags);
+	smp_mb();
 	if (unlikely(list_empty(&shost->free_list))) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&shost->free_list_lock, flags);
 		list_add(&cmd->list, &shost->free_list);
+		spin_unlock_irqrestore(&shost->free_list_lock, flags);
 		cmd = NULL;
 	}
-	spin_unlock_irqrestore(&shost->free_list_lock, flags);
 
 	if (likely(cmd != NULL))
 		scsi_pool_free_command(shost->cmd_pool, cmd);
-- 
1.6.3.rc0.1.gf800

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