[PATCH 13/13] hpsa: remove spin lock around command allocation

[Don Brace]

From: Stephen M. Cameron <stephenmcame...@gmail.com>

It is already using atomic test_and_set_bit to do the
allocation.

There is some microscopic chance of starvation, but it is
so microscopic that it should never happen in reality.

Signed-off-by: Don Brace <don.br...@pmcs.com>
Reviewed-by: Webb Scales <web...@hp.com>
---
 drivers/scsi/hpsa.c |   36 +++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 13 deletions(-)

diff --git a/drivers/scsi/hpsa.c b/drivers/scsi/hpsa.c
index 72daca9..8a3d0de 100644
--- a/drivers/scsi/hpsa.c
+++ b/drivers/scsi/hpsa.c
@@ -4607,19 +4607,32 @@ static struct CommandList *cmd_alloc(struct ctlr_info 
*h)
        int i;
        union u64bit temp64;
        dma_addr_t cmd_dma_handle, err_dma_handle;
-       unsigned long flags;
+       int loopcount;
+
+       /* There is some *extremely* small but non-zero chance that that
+        * multiple threads could get in here, and one thread could
+        * be scanning through the list of bits looking for a free
+        * one, but the free ones are always behind him, and other
+        * threads sneak in behind him and eat them before he can
+        * get to them, so that while there is always a free one, a
+        * very unlucky thread might be starved anyway, never able to
+        * beat the other threads.  In reality, this happens so
+        * infrequently as to be indistinguishable from never.
+        */
 
-       spin_lock_irqsave(&h->lock, flags);
+       loopcount = 0;
        do {
                i = find_first_zero_bit(h->cmd_pool_bits, h->nr_cmds);
-               if (i == h->nr_cmds) {
-                       spin_unlock_irqrestore(&h->lock, flags);
-                       return NULL;
-               }
-       } while (test_and_set_bit
-                (i & (BITS_PER_LONG - 1),
-                 h->cmd_pool_bits + (i / BITS_PER_LONG)) != 0);
-       spin_unlock_irqrestore(&h->lock, flags);
+               if (i == h->nr_cmds)
+                       i = 0;
+               loopcount++;
+       } while (test_and_set_bit(i & (BITS_PER_LONG - 1),
+                 h->cmd_pool_bits + (i / BITS_PER_LONG)) != 0 &&
+               loopcount < 10);
+
+       /* Thread got starved?  We do not expect this to ever happen. */
+       if (loopcount >= 10)
+               return NULL;
 
        c = h->cmd_pool + i;
        memset(c, 0, sizeof(*c));
@@ -4679,13 +4692,10 @@ static struct CommandList *cmd_special_alloc(struct 
ctlr_info *h)
 static void cmd_free(struct ctlr_info *h, struct CommandList *c)
 {
        int i;
-       unsigned long flags;
 
        i = c - h->cmd_pool;
-       spin_lock_irqsave(&h->lock, flags);
        clear_bit(i & (BITS_PER_LONG - 1),
                  h->cmd_pool_bits + (i / BITS_PER_LONG));
-       spin_unlock_irqrestore(&h->lock, flags);
 }
 
 static void cmd_special_free(struct ctlr_info *h, struct CommandList *c)

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