On 06/11/18 14:53, Lorenzo Pieralisi wrote:
> [CC Marc]
>
> On Sat, Oct 27, 2018 at 12:00:57AM +0000, Trent Piepho wrote:
>> This reverts commit 8c934095fa2f ("PCI: dwc: Clear MSI interrupt status
>> after it is handled, not before").
>>
>> This is a very real race that we observed quickly after switching from
>> 4.13 to 4.16. Using a custom PCI-e endpoint and driver, I was able to
>> track it to the precise race and verify the fixed behavior, as will be
>> described below.
>>
>> This bug was originally fixed in 2013, in commit ca1658921b63 ("PCI:
>> designware: Fix missing MSI IRQs") The discussion of that commit,
>> archived in patchwork [1], is informative and worth reading.
>>
>> The bug was re-added in the '8c934 commit this is reverting, which
>> appeared in the 4.14 kernel.
>>
>> Unfortunately, Synopsys appears to consider the operation of this PCI-e
>> controller secret. They provide no publicly available docs for it nor
>> allow the references manuals of SoCs using the controller to publish any
>> documentation of it.
>>
>> So I can not say certain this code is correctly using the controller's
>> features. However, extensive testing gives me high confidence in the
>> accuracy of what is described below.
>>
>> If an MSI is received by the PCI-e controller while the status register
>> bit for that MSI is set, the PCI-e controller will NOT generate another
>> interrupt. In addition, it will NOT queue or otherwise mark the
>> interrupt as "pending", which means it will NOT generate an interrupt
>> when the status bit is unmasked.
>>
>> This gives the following race scenario:
>>
>> 1. An MSI is received by, and the status bit for the MSI is set in, the
>> DWC PCI-e controller.
>> 2. dw_handle_msi_irq() calls a driver's registered interrupt handler
>> for the MSI received.
>> 3. At some point, the interrupt handler must decide, correctly, that
>> there is no more work to do and return.
>> 4. The hardware generates a new MSI. As the MSI's status bit is still
>> set, this new MSI is ignored.
>> 6. dw_handle_msi_irq() unsets the MSI status bit.
>>
>> The MSI received at point 4 will never be acted upon. It occurred after
>> the driver had finished checking the hardware status for interrupt
>> conditions to act on. Since the MSI status was masked, it does not
>> generated a new IRQ, neither when it was received nor when the MSI is
>> unmasked.
>>
>> It seems clear there is an unsolvable race here.
>>
>> After this patch, the sequence becomes as follows:
>>
>> 1. An MSI is received and the status bit for the MSI is set in the
>> DWC PCI-e controller.
>> 2. dw_handle_msi_irq() clears this MSI status bit.
>> 3. dw_handle_msi_irq() calls a driver's registered interrupt handler
>> for the MSI received.
>> 3. At some point, the interrupt handler must decide, correctly, that
>> there is no more work to do and return.
>> 4. The hardware generates a new MSI. This sets the MSI status bit and
>> triggers another interrupt in the interrupt controller(s) above the DWC
>> PCI-e controller. As the the dwc-pcie handler is not re-entrant, it is
>> not run again at this time.
>> 6. dw_handle_msi_irq() finishes. The MSI status bit remains set.
>> 7. The IRQ is re-raised and dw_handle_msi_irq() runs again.
>> 8. dw_handle_msi_irq() invokes the MSI's registered interrupt handler
>> again as the status bit was still set.
>
> Not sure why (5) is not used in your lists, I assume because you want
> to highlight the race condition with the jump from 4 to 6 (or maybe
> you do not like number 5 :), just curious).
>
>> The observant will notice that point 4 present the opportunity for the
>> SoC's interrupt controller to lose the interrupt in the same manner as
>> the bug in this driver. The driver for that interrupt controller will
>> be written to properly deal with this. In some cases the hardware
>> supports an EOI concept, where the 2nd IRQ is masked and internally
>> queued in the hardware, to be re-raised at EOI in step 7. In other
>> cases the IRQ will be unmasked and re-raised at step 4, but the kernel
>> will see the handler is INPROGRESS and not re-invoke it, and instead set
>> a PENDING flag, which causes the handler to re-run at step 7.
>>
>> [1] https://patchwork.kernel.org/patch/3333681/
>>
>> Fixes: 8c934095fa2f ("PCI: dwc: Clear MSI interrupt status after it is handled, not before")
>
> I have two questions:
>
> - Commit 8c934095fa2f has been in the kernel for a year and no
> regression was reported. It was assumed to fix a problem so before
> reverting it I want to make sure we are not breaking anything else.
> - Your reasoning seems correct but I would pick Marc's brain on this
> because I want to understand if what this patch does is what IRQ core
> expects it to do, especially in relation to the IRQ chaining you
> are mentioning.
It is hard to decide what the right solution is without understanding
exactly what this particular write actually does. It seems to be some
form of acknowledgement, but I'm only making an educated guess, and some
of the defines suggest that there might be another register for that.
What I'm interested in is the relationship this has with the mask/unmask
callbacks, and whether masking the interrupt before acking it would help.
Gustavo, can you help here?
In any way, moving the action of acknowledging the interrupt to its
right spot in the kernel (dw_pci_bottom_ack) would be a good start.
Thanks,
M.
--
Jazz is not dead. It just smells funny...
