Re: [PATCH v2] target: core: remove from tmr_list at lun unlink

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On 4/16/21 4:21 AM, Dmitry Bogdanov wrote:
> Currently TMF commands are removed from de_device.dev_tmf_list at
> the very end of se_cmd lifecycle. But se_lun unlinks from se_cmd
> up on a command status (response) is queued in transport layer.
> It means that LUN and backend device can be deleted meantime and at
> the moment of repsonse completion a panic is occured:
> 
> target_tmr_work()
> 	cmd->se_tfo->queue_tm_rsp(cmd); // send abort_rsp to a wire
> 	transport_lun_remove_cmd(cmd) // unlink se_cmd from se_lun
> - // - // - // -
> <<<--- lun remove
> <<<--- core backend device remove
> - // - // - // -
> qlt_handle_abts_completion()
>   tfo->free_mcmd()
>     transport_generic_free_cmd()
>       target_put_sess_cmd()
>         core_tmr_release_req() {
>           if (dev) { // backend device, can not be null
>             spin_lock_irqsave(&dev->se_tmr_lock, flags); //<<<--- CRASH
> 

Sorry about this. I was thinking about this patch some more while
reviewing this version.

Is there another race possible?

1. We have a cmd running in lio.
2. The initiator times it out and sends tmf.
3. cmd completes.
4. target_submit_tmr has called transport_lookup_tmr_lun.
5. transport_lookup_tmr_lun has set se_dev and tmr_dev.
6. You now remove the lun and device.
7. Now you crash on the dev references.

I think we need to do a percpu_ref_tryget_live in transport_lookup_tmr_lun
like is done in transport_lookup_cmd_lun. If successful set

se_cmd->lun_ref_active = true;

If we get the ref, we drop it in transport_lun_remove_cmd like with
the non tmr case.

Combined with this patch then I think we are ok for all races.



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