This patch fixes the following bugs:
1. If there are multiple ordered cmds queued and multiple simple cmds
completing, target_restart_delayed_cmds() could be called on different
CPUs and each instance could start a ordered cmd. They could then run in
different orders than they were queued.
2. target_restart_delayed_cmds() and target_handle_task_attr() can race
where:
1. target_handle_task_attr() has passed the simple_cmds == 0 check.
2. transport_complete_task_attr() then decrements simple_cmds to 0.
3. transport_complete_task_attr() runs target_restart_delayed_cmds() and
it does not see any cmds on the delayed_cmd_list.
4. target_handle_task_attr() adds the cmd to the delayed_cmd_list.
The cmd will then end up timing out.
3. If we are sent > 1 ordered cmds and simple_cmds == 0, we can execute
them out of order, because target_handle_task_attr() will hit that
simple_cmds check first and return false for all ordered cmds sent.
4. We run target_restart_delayed_cmds() after every cmd completion, so if
there is more than 1 simple cmd running, we start executing ordered cmds
after that first cmd instead of waiting for all of them to complete.
5. Ordered cmds are not supposed to start until HEAD OF QUEUE and all older
cmds have completed, and not just simple.
6. It's not a bug but it doesn't make sense to take the delayed_cmd_lock
for every cmd completion when ordered cmds are almost never used. Just
replacing that lock with an atomic increases IOPs by up to 10% when
completions are spread over multiple CPUs and there are multiple
sessions/ mqs/thread accessing the same device.
This patch moves the queued delayed handling to a per device work to
serialze the cmd executions for each device and adds a new counter to track
HEAD_OF_QUEUE and SIMPLE cmds. We can then check the new counter to
determine when to run the work on the completion path.
Link: https://lore.kernel.org/r/20210930020422.92578-3-michael.christie@oracle.com
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
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