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

Don Brace
2014-11-14T23:25:55Z

From: Stephen M. Cameron <stephenmcameron@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.brace@pmcs.com>
Reviewed-by: Webb Scales <webbnh@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)