Re: [PATCH v4] scsi: ufs: Cleanup completed request without interrupt notification

[Can Guo]

Hi Bart,

On 2020-07-31 12:06, Bart Van Assche wrote:
> On 2020-07-30 18:30, Stanley Chu wrote:
> > On Mon, 2020-07-27 at 11:18 +0000, Avri Altman wrote:
> > > Looks good to me.
> > > But better wait and see if Bart have any further reservations.
> >
> > Would you have any further suggestions?
>
> Today is the first time that I took a look at ufshcd_abort(). The
> approach of that function looks wrong to me. This is how I think that a
> SCSI LLD abort handler should work:
> (1) Serialize against the completion path
> (__ufshcd_transfer_req_compl()) such that it cannot happen that the
> abort handler and the regular completion path both call
> cmd->scsi_done(cmd) at the same time. I'm not sure whether an existing
> synchronization object can be used for this purpose or whether a new
> synchronization object has to be introduced to serialize scsi_done()
> calls from __ufshcd_transfer_req_compl() and ufshcd_abort().
> (2) While holding that synchronization object, check whether the SCSI
> command is still outstanding. If so, submit a SCSI abort TMR to the 
> device.
> (3) If the command has been aborted, call scsi_done() and return
> SUCCESS. If aborting failed and the command is still in progress, 
> return
> FAILED.
>
> An example is available in srp_abort() in
> drivers/infiniband/ulp/srp/ib_srp.c.
>
> Bart.


AFAIK, sychronization of scsi_done is not a problem here, because scsi 
layer
use the atomic state, namely SCMD_STATE_COMPLETE, of a scsi cmd to 
prevent
the concurrency of abort and real completion of it.

Check func scsi_times_out(), hope it helps.

enum blk_eh_timer_return scsi_times_out(struct request *req)
{
...
        if (rtn == BLK_EH_DONE) {
                /*
                 * Set the command to complete first in order to prevent 
a real
                 * completion from releasing the command while error 
handling
                 * is using it. If the command was already completed, 
then the
                 * lower level driver beat the timeout handler, and it 
is safe
                 * to return without escalating error recovery.
                 *
                 * If timeout handling lost the race to a real 
completion, the
                 * block layer may ignore that due to a fake timeout 
injection,
                 * so return RESET_TIMER to allow error handling another 
shot
                 * at this command.
                 */
                if (test_and_set_bit(SCMD_STATE_COMPLETE, &scmd->state))
                        return BLK_EH_RESET_TIMER;
                if (scsi_abort_command(scmd) != SUCCESS) {
                        set_host_byte(scmd, DID_TIME_OUT);
                        scsi_eh_scmd_add(scmd);
                }
        }
}

Thanks,

Can Guo.