netdev - Re: Regression in v5.16-rc1: Timeout in mlx5_health_wait_pci

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Date:   Sat, 20 Nov 2021 18:38:37 +0200
From:   Moshe Shemesh <moshe@...dia.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Amir Tzin <amirtz@...dia.com>,
        Saeed Mahameed <saeedm@...dia.com>
CC:     netdev <netdev@...r.kernel.org>, <regressions@...ts.linux.dev>,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: Regression in v5.16-rc1: Timeout in mlx5_health_wait_pci_up() may
 try to wait 245 million years

Thank you for reporting Niklas.

This is actually a case of use after free, as following that patch the 
recovery flow goes through mlx5_tout_cleanup() while timeouts structure 
is still needed in this flow.

We know the root cause and will send a fix.


Thanks,

Moshe.

On 11/19/2021 12:58 PM, Niklas Schnelle wrote:
> Hello Amir, Moshe, and Saeed,
>
> (resent due to wrong netdev mailing list address, sorry about that)
>
> During testing of PCI device recovery, I found a problem in the mlx5
> recovery support introduced in v5.16-rc1 by commit 32def4120e48
> ("net/mlx5: Read timeout values from DTOR"). It follows my analysis of
> the problem.
>
> When the device is in an error state, at least on s390 but I believe
> also on other systems, it is isolated and all PCI MMIO reads return
> 0xff. This is detected by your driver and it will immediately attempt
> to recovery the device with a mlx5_core driver specific recovery
> mechanism. Since at this point no reset has been done that would take
> the device out of isolation this will of course fail as it can't
> communicate with the device. Under normal circumstances this reset
> would happen later during the new recovery flow introduced in
> 4cdf2f4e24ff ("s390/pci: implement minimal PCI error recovery") once
> firmware has done their side of the recovery allowing that to succeed
> once the driver specific recovery has failed.
>
> With v5.16-rc1 however the driver specific recovery gets stuck holding
> locks which will block our recovery. Without our recovery mechanism
> this can also be seen by "echo 1 > /sys/bus/pci/devices/<dev>/remove"
> which hangs on the device lock forever.
>
> Digging into this I tracked the problem down to
> mlx5_health_wait_pci_up() hangig. I added a debug print to it and it
> turns out that with the device isolated mlx5_tout_ms(dev, FW_RESET)
> returns 774039849367420401 (0x6B...6B) milliseconds and we try to wait
> 245 million years. After reverting that commit things work again,
> though of course the driver specific recovery flow will still fail
> before ours can kick in and finally succeed.
>
> Thanks,
> Niklas Schnelle
>
> #regzbot introduced: 32def4120e48
>