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Bug#1141355: linux-image-6.1.0-49-amd64: procfs deadlock triggered by drop_caches on 6.1.0-49-amd64 (regression from 6.1.0-37)

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On Fri, 2026-07-03 at 11:29 +0200, Julian Rilli wrote:
> Package: src:linux
> Version: 6.1.174-1
> Severity: important
> 
> Dear Maintainer,
> 
> After booting into 6.1.0-49-amd64, commands that access /proc or the
> kernel ring buffer (dmesg -T, ps -ef, systemctl status) block
> indefinitely, making the SSH session unresponsive.

How long did you actually wait, approximately?

> The issue is triggered by:
>   sync; echo 3 > /proc/sys/vm/drop_caches
> (run via cronjob every hour)
> 
> This is a regression. Kernel 6.1.0-37-amd64 ran for 396 days with the
> identical cronjob and configuration without any issues.
> 
> Steps to reproduce:
>   1. Boot 6.1.0-49-amd64
>   2. Run: sync; echo 3 > /proc/sys/vm/drop_caches
>   3. Run: dmesg -T (or ps -ef, or systemctl status ssh)
>   4. Command blocks indefinitely, SSH session becomes unresponsive
> 
> Expected: Commands complete normally (as with 6.1.0-37)
> Actual: Commands block, require new SSH session or reboot
> 
> Workaround: Boot 6.1.0-37-amd64 or disable drop_caches cronjob.

vm.drop_caches is an API for testing and not for production use, so I
think it is a good idea to disable that cron job anyway.


[...]
> -- Package-specific info:
> ** Kernel log: boot messages should be attached
[...]

Please provide a kernel log showing use of drop_caches and the
subsequent hang.

Also, the next version (package version 6.1.176-1, kernel version string
6.1.0-50-amd64) will be available very soon, so please test whether that
fixes this for you.

Ben.

-- 
Ben Hutchings
You can't have everything.  Where would you put it?

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Bug#1141355: linux-image-6.1.0-49-amd64: procfs deadlock triggered by drop_caches on 6.1.0-49-amd64 (regression from 6.1.0-37) Julian Rilli <julian@rilli.eu> - 2026-07-03 11:50 +0200
  Processed: Re: Bug#1141355: linux-image-6.1.0-49-amd64: procfs  deadlock triggered by drop_caches on 6.1.0-49-amd64 (regression from  6.1.0-37) "Debian Bug Tracking System" <owner@bugs.debian.org> - 2026-07-03 20:40 +0200
  Bug#1141355: linux-image-6.1.0-49-amd64: procfs deadlock triggered by drop_caches on 6.1.0-49-amd64 (regression from 6.1.0-37) Ben Hutchings <ben@decadent.org.uk> - 2026-07-03 20:40 +0200

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