Christoph, I don't see how the new tagging code with blk-mq can
be providing compatible behavior for existing SCSI drivers.
Do you know that scsi_populate_tag_msg() is always going to provide a
tag? Unconditionally, every time, when using blk-mq?
This is because the test:
if (blk_rq_tagged(req))
is always true if a blk-mq context is attached to the request.
And every scsi request will have a blk-mq context.
scsi_execute() {
req = blk_get_request(sdev->request_queue, write, __GFP_WAIT);
blk_get_request() {
if (q->mq_ops)
return blk_mq_alloc_request(q, rw, gfp_mask, false);
blk_mq_alloc_request() {
rq = __blk_mq_alloc_request(&alloc_data, rw);
__blk_mq_alloc_request() {
blk_mq_rq_ctx_init() {
rq->mq_ctx = ctx;
and therefore:
#define blk_rq_tagged(rq) \
((rq)->mq_ctx || ((rq)->cmd_flags & REQ_QUEUED))
will always evaluate to true.
This cannot work, because we can't put a TAG into the initial SCSI bus
scanning messages for example.
It is a common pattern in SCSI drivers to see if scsi_populate_tag_msg()
succeeds and gives us a non-zero tag, to determine whether to enable
DISCONNECT, whether to send the TAG in the initial MESSAGE_OUT phase
after selection, etc.
It has to be conditional upon the SCSI tcq state, perhaps a test on
whether the queue_depth has been set to a non-zero value.
scsi_populate_tag_msg() is giving out tags even when
sdev->tagged_supported is false, which is bogus.
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