On Thu, Aug 06, 2020 at 08:07:38PM +0530, Kashyap Desai wrote:

quoted

quoted

Hi Ming -

There is still some race which is not handled.  Take a case of IO is
not able to get budget and it has already marked <restarts> flag.
<restarts> flag will be seen non-zero in completion path and
completion path will attempt h/w queue run. (But this particular IO is
still not in s/w queue.).
Attempt of running h/w queue from completion path will not flush any
IO since there is no IO in s/w queue.

Then where is the IO to be submitted in case of running out of budget?

Typical race in your latest patch is - (Lets consider command A,B and C)
Command A did not receive budget. Command B completed  (which was already

Command A doesn't get budget, and A is still in sw/scheduler queue
because we try to acquire budget before dequeuing request from sw/scheduler queue,
see __blk_mq_do_dispatch_sched() and blk_mq_do_dispatch_ctx().

Not consider direct issue, because the hw queue will be run explicitly
when not getting budget, see __blk_mq_try_issue_directly.

Not consider command A being added to hctx->dispatch too, because blk-mq will
re-run the queue, see blk_mq_dispatch_rq_list().

submitted earlier) at the same time and it make sdev->device_busy = 0 from
" scsi_finish_command".
Command B has still not called "scsi_end_request". Command C get the
budget and it will make sdev->device_busy = 1. Now, Command A set  set
sdev->restarts flags but will not run h/w queue since sdev->device_busy =
1.

Right.

Command B run h/w queue (make sdev->restart = 0) from completion path, but
command -A is still not in the s/w queue.

Then you didn't answer my question about where A is, did you?

Command-A is in now in s/w queue. Command-C completed but it will not run h/w queue because
sdev->restarts = 0.

Why does command-A become in sw/queue now?

No.


Thanks,
Ming