Re: [PATCH] Bio Traversal Changes

From: James Bottomley (
Date: Fri Aug 02 2002 - 08:48:06 EST

The SCSI changes (small that they are) look reasonable.

This does look like it exposes an existing problem in the tag/barrier
approach, though.

The bio can be split by making multiple requests over segements of the bio,
correct? If this is a BIO_RW_BARRIER, then each of these requests will be a
REQ_BARRIER. However, in the SCSI paradigm where we translate REQ_BARRIER to
ordered tag, each of the requests will get a new ordered tag as it comes back
around through end_that_request_first, potentially allowing other tags to be
inserted in between these, which would be incorrect, since other bios would be
inserted in between the segments of this one, thus violating the barrier.

Is the above correct? If it is, I may have finally found a use for linked
scsi tasks (gives you the ability to have one tag cover multiple commands).


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