[Scst-devel] Understanding SCST internals, a BUG ?

[Anton N. <ano...@gm...>]

I'm looking for a help in understanding of SCST internal architecture
and operation. The problem
I'm experiencing now is that SCST seems to process deferred commands
incorrectly in some cases.
More specifically, I'm confused with the 'while' loop in
scst_send_to_midlev function.

As far as I understand, the basic execution path consists of a call to
scst_do_send_midlev
followed by taking of a decision on this command (continue with this
command, reschedule it,
or move to the next one), the decision is stored in 'int res', which
is then returned from
the function.

However, if there are deferred commands on the device, the function
does not return but
makes another call to scst_do_send_to_midlev, analyzes the return code
again and stores the
decision in 'int res' thereby erasing the decision for the previous
command. If scst_send_to_midlev
exits now, it will return the _new_ decision (for the deferred
command) whereas the scst_process_active_cmd
will think that it is the decision for the command that was originally
passed to scst_send_to_midlev.

For example, this will cause problems in the following situation:
1. scst_send_to_midlev is called with cmd == 0x80000100
2. scst_do_send_to_midlev is called with cmd == 0x8000100
3. scst_do_send_to_midlev returns with SCST_EXEC_COMPLETED
   (in certain scenarios the command is already destroyed at this point)
4. scst_check_deferred_commands finds the defferred cmd == 0x80000200
5. scst_do_send_to_midlev is called with cmd == 0x80000200
6. scst_do_send_to_midlev returns with SCST_EXEC_NEED_THREAD
7. scst_send_to_midlev returns with SCST_CMD_STATE_RES_NEED_THREAD
8. Now, the scst_process_active_cmd will try to reschedule command 0x8000100
   which is already destroyed at this point !

Can anyone on the list confirm my guess? Or, this situation should
never happen because
of some other condition which I may have missed? Right now I can't
think of any of simple
methods to work around the issue, i.e. any of my ideas require
rewriting significant part
of the code.