Re: [PATCH libata-dev-2.6:upstream 00/02] atapi: packet task vs. intr race fix

[Jeff Garzik <jgarzik@pobox.com>]

Tejun Heo wrote:
> Jeff Garzik wrote:
> 
>> Tejun Heo wrote:
>>
>>>
>>>  Hello, Jeff.
>>>
>>> Jeff Garzik wrote:
>>>
>>>> Tejun Heo wrote:
>>>>
>>>>>  Hello, Jeff and Albert.
>>>>>
>>>>>  This patchset fixes the following race.
>>>>> (port A has ATA device and B ATAPI).
>>>>>
>>>>> port B    : ata_issue_prot() (ATAPI_NODATA)
>>>>> port B    : packet_task scheduled
>>>>> port A    : ata_issue_prot() (DMA)
>>>>> intr    : ata_interrupt()
>>>>> port A    : ata_host_intr -> this is all good & dandy
>>>>> port B    : ata_host_intr -> finishes ATAPI cmd w/ error (request 
>>>>> sense)
>>>>>
>>>>>  This is where the race is, we're polling for port B's qc, we must not
>>>>> mess with it from interrupt.  Now, port B has dangling packet_task
>>>>> which will race w/ whatever will run on port B.
>>>>>
>>>>>  The problem is that we don't always protect polled ports from
>>>>> interrupts with ata_qc_set_polling() and for non-DMA ATA commands
>>>>> there's no way to discern if actually an IRQ has occurred (this
>>>>> sucks), so we end up finishing the other port's command.
>>>>>
>>>>>  This condition occurs quite often if both port A and B are busy.  The
>>>>> most common result is assertion failure in atapi_packet_task
>>>>> (assert(qc->flags & ATA_QCFLAG_ACTIVE)), but depending on timing and
>>>>> on SMP, weirder things could happen, I think.
>>>>>
>>>>>  Note that for ATAPI_DMA, interrupt from the other port won't mess
>>>>> with a polled command as we can tell that it's not ours with
>>>>> bmdma_status, but, if spurious interrupt occurs on the port, the
>>>>> packet_task will go dangling.  That's why ATAPI_DMA also needs
>>>>> protection.  The baseline is that all polled qc's need to be protected
>>>>> with ata_qc_set_polling() until polling task is done with the command.
>>>>>
>>>>>
>>>>> [ Start of patch descriptions ]
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> While this is something to look into, the supplied patches are 
>>>> definitely not the way we want to go.  We need to follow the state 
>>>> diagram for the PACKET command protocol.  ATA4 [1] diagrams are a 
>>>> good thing to read, since they include mention of the behavior of 
>>>> certain ATAPI devices that can send an interrupt between 
>>>> taskfile-out and write-cdb steps in the sequence.
>>>>
>>>> In patch #2, you're making ata_irq_on() way too heavy.  In patch #1, 
>>>> calling ata_qc_set_polling() for non-polled commands is a hack.
>>>
>>>
>>>
>>>
>>>  Have you received patches in the reverse order?  #1 changes 
>>> ata_irq_on() and #2 adds ata_qc_set_polling().
>>>
>>>  Hmmm, only a call to ata_chk_status() is added to ata_irq_on(), 
>>> which I think is needed regardless of other changes, and 
>>> ata_wait_idle() is removed.  Does that make the function heavy?
>>>
>>>>
>>>> The better solution is to track the PACKET command protocol state 
>>>> much more closely, so that the code _knows_ when it should and 
>>>> shouldn't be getting an interrupt.
>>>>
>>>> This is required anyway because, as mentioned in another email, an 
>>>> ATA device might assert INTRQ for certain events, such as non-data 
>>>> commands, where the controller is not required to assert the BMDMA 
>>>> IRQ bit.
>>>>
>>>> I _suspect_ that many host controllers cause the BMDMA IRQ bit to 
>>>> track the ATA INTRQ status precisely, but this theory has not been 
>>>> validated.
>>>>
>>>>     Jeff
>>>>
>>>>
>>>> [1] http://www.t13.org/project/d1153r18-ATA-ATAPI-4.pdf
>>>
>>>
>>>
>>>
>>>  Without interrupt pending information from BMDMA bit for non-DMA 
>>> commands (which I don't think we can use for non-DMA cmds as we'll 
>>> never be sure if all controllers behave that way), the problem is 
>>> that for many SATA controllers, more than one ports share single 
>>> interrupt line.   And without interrupt pending bit, shared interrupt 
>>> means a lot of spurious interrupts making it impossible to know when 
>>> to expect interrupts.
>>
>>
>>
>> Incorrect...  this is why I keep harping on the "ATA host state machine."
>>
>> You rely on proper implementation to know when to expect interrupts. 
>> Read the state diagrams, they tell you precisely when an interrupt may 
>> be expected.  By definition, any other interrupt is probably a PCI 
>> shared interrupt, or a hardware or software bug.
>>
> 
>  I think I'm familiar with ATA host state machine.  I don't think I'm 
> currently saying things because I don't know about it.  Maybe I'm 
> missing something here.  Please consider the following scenario.
> 
> 1. non-DMA command issued.
> 2. device does something
> 3. another device sharing the IRQ line raises interrupt
> 4. D2H FIS received (BSY off, DRDY on, INTR on)
> 5. interrupt handler kicks in for #3
> 6. SATA controller raises interrupt
> 7. SATA handler invoked from #5 checks ATA_STATUS and determines that
>    completes the command (we don't know if it's our interrupt or not)
> 8. interrupt handler from #5 returns.
> 9. interrupt handler for #6.
> 10. nobody cared.
> 
>  Did I miss something?

This gets into interrupt controller hardware, how interrupts are 
asserted and delivered, and similar not-IDE-specific things.

If you clear the condition (by checking ATA Status) that caused the 
interrupt hardware to raise an interrupt, during #5 and #7, then the 
interrupt condition is cleared in the interrupt controller hardware, and 
the kernel should not invoke the interrupt handler again.


>  I think the problem is whether or not we're expecting interrupts, we'll 
> get interrupts that are't ours and we have no way to tell if they're 
> ours or not.  No?

You can tell just by following normal interrupt assertion rules, and by 
the "if it's nor ours it's either somebody else's interrupt, or 
out-of-spec hardware"

In addition, (slight tangent)
* if DMA is active, you can check the DMA status bits to see if you have 
an interrupt condition to handle
* if DMA is not active, you can check the AltStatus register, to test 
the state without actually clearing anything

Though beware, on rare PATA controllers (old Macs?) the AltStatus 
register doesn't exist.

	Jeff