scsi/sd: remove big kernel lock

Every user of the BKL in the sd driver is the
result of the pushdown from the block layer
into the open/close/ioctl functions.

The only place that used to rely on the BKL is
the sdkp->openers variable, which gets converted
into an atomic_t.

Nothing else seems to rely on the BKL, since the
functions do not touch global data without holding
another lock, and the open/close functions are
still protected from concurrent execution using
the bdev->bd_mutex.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: linux-scsi@vger.kernel.org
Cc: "James E.J. Bottomley" <James.Bottomley@suse.de>
Acked-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

2 files changed, 8 insertions, 11 deletions

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 01680c7c850..fc5d69a84af 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -783,6 +783,8 @@ static int sd_prep_fn(struct request_queue *q, struct request *rq)
  *      or from within the kernel (e.g. as a result of a mount(1) ).
  *      In the latter case @inode and @filp carry an abridged amount
  *      of information as noted above.
+ *
+ *      Locking: called with bdev->bd_mutex held.
  **/
 static int sd_open(struct block_device *bdev, fmode_t mode)
 {
@@ -795,7 +797,6 @@ static int sd_open(struct block_device *bdev, fmode_t mode)
 
         SCSI_LOG_HLQUEUE(3, sd_printk(KERN_INFO, sdkp, "sd_open\n"));
 
-        lock_kernel();
         sdev = sdkp->device;
 
         /*
@@ -834,17 +835,15 @@ static int sd_open(struct block_device *bdev, fmode_t mode)
         if (!scsi_device_online(sdev))
                 goto error_out;
 
-        if (!sdkp->openers++ && sdev->removable) {
+        if ((atomic_inc_return(&sdkp->openers) == 1) && sdev->removable) {
                 if (scsi_block_when_processing_errors(sdev))
                         scsi_set_medium_removal(sdev, SCSI_REMOVAL_PREVENT);
         }
 
-        unlock_kernel();
         return 0;
 
 error_out:
         scsi_disk_put(sdkp);
-        unlock_kernel();
         return retval;  
 }
 
@@ -858,6 +857,8 @@ error_out:
  *
  *      Note: may block (uninterruptible) if error recovery is underway
  *      on this disk.
+ *
+ *      Locking: called with bdev->bd_mutex held.
  **/
 static int sd_release(struct gendisk *disk, fmode_t mode)
 {
@@ -866,8 +867,7 @@ static int sd_release(struct gendisk *disk, fmode_t mode)
 
         SCSI_LOG_HLQUEUE(3, sd_printk(KERN_INFO, sdkp, "sd_release\n"));
 
-        lock_kernel();
-        if (!--sdkp->openers && sdev->removable) {
+        if (atomic_dec_return(&sdkp->openers) && sdev->removable) {
                 if (scsi_block_when_processing_errors(sdev))
                         scsi_set_medium_removal(sdev, SCSI_REMOVAL_ALLOW);
         }
@@ -877,7 +877,6 @@ static int sd_release(struct gendisk *disk, fmode_t mode)
          * XXX is followed by a "rmmod sd_mod"?
          */
         scsi_disk_put(sdkp);
-        unlock_kernel();
         return 0;
 }
 
@@ -930,7 +929,6 @@ static int sd_ioctl(struct block_device *bdev, fmode_t mode,
         SCSI_LOG_IOCTL(1, printk("sd_ioctl: disk=%s, cmd=0x%x\n",
                                                 disk->disk_name, cmd));
 
-        lock_kernel();
         /*
          * If we are in the middle of error recovery, don't let anyone
          * else try and use this device.  Also, if error recovery fails, it
@@ -960,7 +958,6 @@ static int sd_ioctl(struct block_device *bdev, fmode_t mode,
                         break;
         }
 out:
-        unlock_kernel();
         return error;
 }
 
@@ -2346,7 +2343,7 @@ static int sd_probe(struct device *dev)
         sdkp->driver = &sd_template;
         sdkp->disk = gd;
         sdkp->index = index;
-        sdkp->openers = 0;
+        atomic_set(&sdkp->openers, 0);
         sdkp->previous_state = 1;
 
         if (!sdp->request_queue->rq_timeout) {
diff --git a/drivers/scsi/sd.h b/drivers/scsi/sd.h
index 43d3caf268e..f81a9309e6d 100644
--- a/drivers/scsi/sd.h
+++ b/drivers/scsi/sd.h
@@ -47,7 +47,7 @@ struct scsi_disk {
         struct scsi_device *device;
         struct device   dev;
         struct gendisk  *disk;
-        unsigned int    openers;        /* protected by BKL for now, yuck */
+        atomic_t        openers;
         sector_t        capacity;       /* size in 512-byte sectors */
         u32             index;
         unsigned short  hw_sector_size;