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When updating `watchdog_thresh`, there is a race condition between writing the new `watchdog_thresh` value and stopping the old watchdog timer. If the old timer triggers during this window, it may falsely detect a softlockup due to the old interval and the new `watchdog_thresh` value being used. The problem can be described as follow: # We asuume previous watchdog_thresh is 60, so the watchdog timer is # coming every 24s. echo 10 > /proc/sys/kernel/watchdog_thresh (User space) | +------>+ update watchdog_thresh (We are in kernel now) | | # using old interval and new `watchdog_thresh` +------>+ watchdog hrtimer (irq context: detect softlockup) | | +-------+ | | + softlockup_stop_all To fix this problem, introduce a shadow variable for `watchdog_thresh`. The update to the actual `watchdog_thresh` is delayed until after the old timer is stopped, preventing false positives. The following testcase may help to understand this problem. --------------------------------------------- echo RT_RUNTIME_SHARE > /sys/kernel/debug/sched/features echo -1 > /proc/sys/kernel/sched_rt_runtime_us echo 0 > /sys/kernel/debug/sched/fair_server/cpu3/runtime echo 60 > /proc/sys/kernel/watchdog_thresh taskset -c 3 chrt -r 99 /bin/bash -c "while true;do true; done" & echo 10 > /proc/sys/kernel/watchdog_thresh & --------------------------------------------- The test case above first removes the throttling restrictions for real-time tasks. It then sets watchdog_thresh to 60 and executes a real-time task ,a simple while(1) loop, on cpu3. Consequently, the final command gets blocked because the presence of this real-time thread prevents kworker:3 from being selected by the scheduler. This eventually triggers a softlockup detection on cpu3 due to watchdog_timer_fn operating with inconsistent variable - using both the old interval and the updated watchdog_thresh simultaneously. [nysal@linux.ibm.com: fix the SOFTLOCKUP_DETECTOR=n case] Link: https://lkml.kernel.org/r/20250502111120.282690-1-nysal@linux.ibm.com Link: https://lkml.kernel.org/r/20250421035021.3507649-1-luogengkun@huaweicloud.com Signed-off-by: Luo Gengkun <luogengkun@huaweicloud.com> Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.com> Cc: Doug Anderson <dianders@chromium.org> Cc: Joel Granados <joel.granados@kernel.org> Cc: Song Liu <song@kernel.org> Cc: Thomas Gleinxer <tglx@linutronix.de> Cc: "Nysal Jan K.A." <nysal@linux.ibm.com> Cc: Venkat Rao Bagalkote <venkat88@linux.ibm.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.