[PATCH v6 06/20] irqdomain: Fix mapping-creation race

[Cyril Brulebois]

Hi Johan,

And thanks so much for this patch series.

Johan Hovold <johan+linaro at kernel.org> (2023-02-13):
> Parallel probing of devices that share interrupts (e.g. when a driver
> uses asynchronous probing) can currently result in two mappings for the
> same hardware interrupt to be created due to missing serialisation.
> 
> Make sure to hold the irq_domain_mutex when creating mappings so that
> looking for an existing mapping before creating a new one is done
> atomically.

Just for information: This patch fixes a long-standing regression
regarding Raspberry Pi devices, which have been failing to boot (at
least reliably) due to MMC timeouts for a long while; I think that
started between v5.17 and v5.19, but I couldn't bisect at the time
(I was already chasing some other regression).

Example bug report:
  https://bugs.debian.org/1019700

Before trying to pinpoint when the regression appeared, I've checked
these versions, with a Debian testing userspace as of 2023-03-07:
 - v6.1.12: affected.
 - v6.2: affected.
 - v6.3-rc1: not affected.

A bisect between v6.2 and v6.3-rc1 led me to this patch specifically.
Seeing how it's part of a patch series, and how previous patches are
preliminary ones, I've checked that cherry-picking the first 6 patches
on top of v6.1.15 indeed fixes the problem there too, and it does
(git cherry-pick v6.2-rc4..601363cc08da25747feb87c55573dd54de91d66a).


With the following systems:
 - Pi 4 B, using external storage (SD card),
 - CM4 Lite on CM4 IO Board, using external storage (SD card),
 - CM4 on CM4 IO Board, using internal storage (eMMC),

I've been able to verify that v6.1.12 (baseline in Debian testing)
triggers this MMC timeout issue, while v6.1.15 + the aforementioned
range of cherry-picked commits no longer triggers this issue.

(Methodology: cold boot then reboot 20 times, monitoring via serial
console to keep HDMI output of the equation; affected systems stop
booting after 1-4 boots; unaffected systems boot and reboot just fine
all the time.)


This looks like a critical bugfix for Raspberry Pi users.

Seeing the stable@ mention is about 4.8, I suppose this is going to be
considered for a wide range of kernels already… but I'm happy to dig
into this further to pinpoint when the regression appeared, if that's
helpful.


Cheers,
-- 
Cyril Brulebois (kibi at debian.org)            <https://debamax.com/>
D-I release manager -- Release team member -- Freelance Consultant
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