RE: [patch] scsi: use lock per host instead of per device for shared queue tag host

[David Somayajulu]

> It seems another driver(qla4xxx) is also using shared queue tag.
> It is natural to imagine there might be same symptom in that
> driver. But I don't know the driver and have no hardware so I
> can not say anything certain about it.

qla4xxx implements slightly differently, in the sense we don't have the
equivalent of         
struct st_ccb ccb[MU_MAX_REQUEST]; 
which is in struct st_hba. In other words we don't have a local array
which like stex to keep track of the outstanding commands to the hba.

We had a discussion on this one while implementing block-layer tagging
in qla4xxx and Jens Axboe added the test_and_set_bit() in the following
code in blk_queue_start_tag() to take care of it.
	do {
		tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
		if (tag >= bqt->max_depth)
			return 1;
	} while (test_and_set_bit(tag, bqt->tag_map));
Please see the following link for the discussion
http://marc.theaimsgroup.com/?l=linux-scsi&m=115886351206726&w=2

Cheers
David Somayajulu
QLogic Corporation
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