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Commit 4c413128 authored by Stephen M. Cameron's avatar Stephen M. Cameron Committed by Christoph Hellwig
Browse files

hpsa: remove spin lock around command allocation 



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: default avatarDon Brace <don.brace@pmcs.com>
Reviewed-by: default avatarWebb Scales <webbnh@hp.com>
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
parent 4fa604e1

drivers/scsi/hpsa.c

+23 −13
Original line number Diff line number Diff line
@@ -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);

		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;

		}

	} 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);



	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)