Linux Kernel RCU Warning in S390 Arch CPU Idle Function

A warning related to the Read-Copy-Update (RCU) mechanism has been introduced in the Linux kernel's idle management for the S390 architecture. This warning, which indicates that RCU is not properly engaged during the CPU idle process, could lead to potential issues in managing concurrent operations. The warning arises from a new tracing feature in the cpuidle subsystem, which detected that the arch_cpu_idle function was not under RCU's watch, contrary to expectations for this architecture.

Impact

The lack of proper RCU handling in the arch_cpu_idle function can lead to incorrect synchronization, potentially causing issues in managing concurrent processes or resources on the affected system.

Reproduction

The vulnerability can be reproduced by compiling the Linux kernel with the default configuration for the S390 architecture. Once this kernel is booted, the cpuidle tracing feature will generate a warning indicating that RCU is not watching during the execution of the arch_cpu_idle function. This warning can be seen in the kernel log, highlighting the need for proper RCU management in the idle function.

Remediation

To address this issue, the arch_cpu_idle function should be marked with the noinstr attribute, indicating that it should not be traced for recursion. This change has already been committed to the Linux kernel stable tree.