James Bottomley wrote:
On Thu, 2005-06-02 at 20:31 +0300, Liran Schour wrote:
I am developing a new SCSI upper layer driver. (Object based SCSI device)
I noticed that scsi_do_req execute commands in LIFO order (add new commands
to the head of the Q).
This behavior can cause a starvation of commands (or at least long delays).
What is the proper solution to this?
Well, to use the proper interfaces.
scsi_do_req is deprecated, so don't use it. scsi_wait_req is its
replacement.
James,
scsi_wait_req() doesn't look like a replacement of
scsi_do_req() to me. The "wait" call is synchronous
while scsi_do_req() is asynchronous with a done()
callback.
Note: they're only really intended to allow ULDs to
execute commands that are necessary for controlling the device (that's
why they go at the head of the queue).
The "char" ULDs use scsi_do_req() exclusively for
SCSI command injection. The sg driver could be sharing
the device with sd driver, but then again it could
be talking to a SES target. The block layer SG_IO ioctl
is the only use in "mixed" mode (e.g. smartmontools
usage). However the block layer SG_IO is not being
used in "mixed" mode when a CD/DVD is being burnt.
IMO the mid level should not arbitrarily adopt LIFO
order.
Block commands are executed in
order using ULD init_command() (or REQ_BLOCK_PC).
OSDs break the UNIX block paradigm. Perhaps
it's time to start thinking about how to properly
support them.
Doug Gilbert
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