Re: [PATCH 2/4 v2] libata: Implement disk shock protection support

[Tejun Heo]

Hello, Elias.

Elias Oltmanns wrote:
>>> +	do {
>>> +		unsigned long now;
>>> +
>>> +		deadline = jiffies;
>>> +		ata_port_for_each_link(link, ap) {
>>> +			ata_link_for_each_dev(dev, link) {
>>> +				struct ata_eh_info *ehi = &link->eh_context.i;
>>> +
>>> +				if (dev->class != ATA_DEV_ATA)
>>> +					continue;
>>> +
>>> +				ata_eh_pull_action(link, dev, ATA_EH_PARK);
>>> +				if (ehi->dev_action[dev->devno] & ATA_EH_PARK) {
>>> +					unsigned long tmp =
>>> +						dev->unpark_deadline;
>>> +
>>> +					if (time_before(deadline, tmp))
>>> +						deadline = tmp;
>>> +					else if (time_before_eq(tmp, jiffies))
>>> +						continue;
>>> +				}
>>> +
>>> +				ata_eh_park_issue_cmd(dev, 1);
>>> +			}
>>> +		}
>>> +		now = jiffies;
>>> +		if (time_before_eq(deadline, now))
>>> +			break;
>>> +		prepare_to_wait(&ata_scsi_park_wq, &wait, TASK_UNINTERRUPTIBLE);
>> Doesn't prepare_to_wait() have to come before pull_action and timeout
>> check?  Which in turn means that it should be a completion instead of
>> wait combined with INIT_COMPLETION because thread state can't be used
>> to track wake up as ata_eh_park_issue_cmd() sleeps.
> 
> Very well spotted, I obviously have to get to know more about these
> things.

Yay, even I am right sometimes.  :-)

> Now, even though I believe that I got the general point you are
> making, I'm not quite sure about what you are saying about wait combined
> with INIT_COMPLETION not being the right thing. In fact, that's
> precisely what I'm going to propose as a solution to our problem. Please
> tell me if I got something fundamentally wrong here.
> 
> The thing grew into a rather more complex problem than I had thought at
> first,

The code itself isn't too bad, I think.

> so I'm just resending the whole patch. Please let me know what
> you think.
> 
> @@ -2830,6 +2878,51 @@ int ata_eh_recover(struct ata_port *ap, ata_prereset_fn_t prereset,
>  		}
>  	}
>  
> +	do {
> +		unsigned long now;
> +
> +		ata_eh_pull_park_action(ap);
> +		deadline = jiffies;
> +		ata_port_for_each_link(link, ap) {
> +			ata_link_for_each_dev(dev, link) {
> +				struct ata_eh_context *ehc = &link->eh_context;
> +				unsigned long tmp;
> +
> +				if (dev->class != ATA_DEV_ATA)
> +					continue;
> +				if (!(ehc->i.dev_action[dev->devno] &
> +				      ATA_EH_PARK))
> +					continue;
> +				tmp = dev->unpark_deadline;
> +				if (time_before(deadline, tmp))
> +					deadline = tmp;
> +				else if (time_before_eq(tmp, jiffies))
> +					continue;
> +				if (ehc->unloaded_mask & (1 << dev->devno))
> +					continue;
> +
> +				ata_eh_park_issue_cmd(dev, 1);
> +			}
> +		}
> +
> +		now = jiffies;
> +		if (time_before_eq(deadline, now))
> +			break;
> +
> +		deadline = wait_for_completion_timeout(&ap->park_req_pending,
> +						       deadline - now);
> +	} while (deadline);

This should basically work but completion isn't really designed for
this type of continuous events where single consumption should clear
all pending events.  INIT_COMPLETION comes close but it doesn't lock,
so can't guarantee anything.  What's necessary is the counterpart for
complete_all() for the wait.

Well, anyways, I think the issue is slightly out of scope for this
patch and the only side effect is possibly looping over the do {}
while () block several times unnecessarily on certain cases, so I
think just noting about it should be enough for now.

Can you please add explanation above wait_for_complete_timeout() that
all done counts should be cleared here but aren't and as a result the
loop might repeat determinate number of times unnecessarily and resend
as proper patch?

Thanks for your patience.

-- 
tejun
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