Linux Kernel Deadlock Due to Improper Lock Ordering

Introduction to Kernel-Mode Deadlocks in Linux

Kernel-mode deadlocks in Linux occur when two or more kernel threads or processes are blocked indefinitely, each waiting for a resource held by another. These issues often stem from improper lock ordering, race conditions, or resource allocation anomalies. Deadlocks in the kernel can lead to system-wide hangs, high CPU usage, and instability, making them critical for system administrators and kernel developers to diagnose and resolve.

Symptoms of a Kernel-Mode Deadlock

• Processes enter D-state (uninterruptible sleep) and become unresponsive
• High CPU utilization with no apparent process activity
• Kernel logs showing “BUG: kernel deadlock” or “spinlock deadlock” messages
• System “hangs” during I/O operations or under load

Root Cause Analysis

Deadlocks typically arise from incorrect lock acquisition order. For example, if a kernel module acquires lock A then lock B, while another thread acquires lock B then lock A, a circular dependency can form. This is exacerbated by the absence of lock dependency tracking mechanisms or improper use of lock classes in the kernel’s locking subsystem. The Linux kernel’s lockdep feature is designed to detect such issues, but misconfigurations or incompatible code paths can bypass its checks.

Example Code Illustrating the Deadlock

Consider the following hypothetical kernel module code snippet:

spinlock_t lock_a, lock_b;

void thread1() {

spin_lock(&lock_a);
spin_lock(&lock_b);
/* critical section */
spin_unlock(&lock_b);
spin_unlock(&lock_a);

}

void thread2() {

spin_lock(&lock_b);
spin_lock(&lock_a);
/* critical section */
spin_unlock(&lock_a);
spin_unlock(&lock_b);

}

Diagnosis Tools and Techniques

• dmesg: Check for kernel panic or lockdep warnings
• perf: Profile CPU usage and identify stalled threads
• ftrace: Trace function calls to detect lock acquisition sequences
• ps and top: Identify processes in D-state or high-CPU states
• lockdep: Enable kernel lock dependency tracking to detect ordering violations

Step-by-Step Solution

1. Enable CONFIG_LOCKDEP in the kernel configuration and rebuild the kernel to activate lock dependency tracking.
2. Analyze dmesg output for deadlock-related messages, such as lockdep: lock blocked: ....
3. Use perf to capture CPU stalls and identify which threads are waiting on locks:

perf record -g -p <pid> perf stat -d -p <pid>

4. Run ftrace to trace lock acquisition paths:

trace-cmd record -p function -s -f spin_lock

5. Examine the trace output to determine the order of lock acquisitions and identify circular dependencies.
6. Modify the code to enforce consistent lock ordering across all paths. For example, always acquire lock_a before lock_b.
7. Recompile the kernel module or kernel, and test under load to confirm the deadlock is resolved.