Preventing And Recovering From Kernel Deadlocks

Understanding Kernel Deadlocks

A kernel deadlock is a state where two or more processes are unable to proceed because each is waiting on another to release a needed resource. This creates a circular dependency that brings the kernel to a halt. Kernel deadlocks are caused by inconsistent lock ordering, race conditions, priority inversions, and resource starvation.

The main symptoms of a kernel deadlock are an unresponsive system and processes in an uninterruptible sleep state. Key impacts include system downtime, data loss or corruption, and revenue impacts for businesses. Preventing deadlocks through careful resource management is key to maintaining reliability.

Defining Kernel Deadlocks and What Causes Them

A kernel deadlock occurs when two or more kernel threads or processes become blocked, each waiting on the other to release needed resources. This creates a circular dependency that brings the kernel to a halt. Typical causes include:

Table of Contents

Inconsistent lock ordering – locks are taken in different orders by different execution paths
Race conditions – contention for resources without proper synchronization
Priority inversions – a high priority process is blocked waiting on a lower priority process
Resource starvation – a process is unable to obtain resources due to excessive contention

These conditions allow kernels threads to become stuck in uninterruptible sleep states as they wait for resources in use by other blocked threads. The end result is a deadlocked kernel unable to performuseful work.

Identifying Symptoms of a Kernel Deadlock

The main symptoms exhibited by systems experiencing kernel deadlocks include:

An unresponsive, hung, or frozen system
Processes stuck in uninterruptible (D) states
Heavy disk I/O at times system appears hung
Kernel log or console messages about held locks
Mount failures, I/O errors, or request timeouts

Checking running processes with utilities like ps will show several in uninterruptible sleep states, indicating they are blocked and waiting on resources. The system log may also contain warnings about held locks and blocking chains. These symptoms signify threads are deadlocked and unable to continue.

Explaining the Impacts of Kernel Deadlocks

The effects of kernel deadlocks range from inconvenient to catastrophic depending on the purpose of the system. Typical impacts include:

Downtime and unavailability of services
Failure to meet service level objectives for uptime/availability
Delayed processing and transactions, backlogs
Data loss or corruption if buffers and caches overflow
Crashes or forced reboots leading to improper shutdowns
Revenue/productivity losses from downtime

Userspace processes may also become blocked or crash due to their dependence on a deadlocked kernel. In mission critical environments like data centers even brief outages incur significant financial costs. Preventing deadlocks is key for reliability.

Preventing Kernel Deadlocks

Strategies for preventing kernel deadlocks center on careful resource management, synchronization, and lock ordering discipline:

Allocate resources up front during initialization
Always acquire locks in a fixed, consistent order
Avoid race conditions with proper synchronization
Use lockdep or tools to detect inconsistent lock usage
Scale resources to match demand, reducing contention

Well-designed locking disciplines eliminate circular wait dependencies. Runtime tools like lockdep help detect and prevent inconsistent lock handling.

Careful Resource Allocation

Deadlocks often result from poor resource allocation policies and programming errors. Best practices include:

Allocate needed resources like memory, files, and mutexes during initialization to prevent unavailable resource deadlocks.
Size resources to anticipated peak demand to reduce contention.
Implement upper bounds on allocations to prevent resource exhaustion.
Use semaphores or other synchronization objects when consumable resources cannot be fully preallocated.

Following initialization best practices prevents “out of resource” deadlock scenarios.

Avoiding Race Conditions and Inconsistent Locking

Race conditions and inconsistent lock handling occur when multiple threads access shared resources without adequate synchronization. This can lead to circular blocking chains. Solutions include:

Identify all shared resources and use kernel primitives like spinlocks and mutexes to serialize access.
Avoid holding locks during long operations – move work outside critical sections.
Use lockdep or dynamic lock analysis to detect inconsistent lock handling.
Adhere to layered locking disciplines, e.g. taking FS locks before lower level disk locks.

Proper locking prevents concurrent access anomalies that cause deadlocks.

Example Lock Ordering Code to Prevent Deadlocks

Here is skeleton code illustrating a safe lock ordering rule to prevent deadlocks in filesystem operations:

struct super_block {
  spinlock_t lock;
};

struct inode {
  spinlock_t lock;
};

void operate_on_data(struct inode *inode){

  /* Acquire locks always in address order */
  if(&inode->lock < &inode->sb->lock){
    spin_lock(&inode->lock);
    spin_lock(&inode->sb->lock);
  } else { 
    spin_lock(&inode->sb->lock);
    spin_lock(&inode->lock);    
  }

  /* Perform data operation... */

  spin_unlock(&inode->sb->lock);  
  spin_unlock(&inode->lock);
}

This lock ordering discipline prevents circular wait dependencies between the locks and eliminates deadlocks.

Recovering From Kernel Deadlocks

When deadlocks do occur, options include triggering soft reboots, forced resets, and checking logs to identify root causes. Magic sysrq keys provide emergency controls.

Magic SysRq keys issue reboot commands if deadlocked
Watchdog timers can automatically force resets
Check system logs to identify deadlock root causes
Scale or partition resources to reduce contention probability

Using Magic Sysrq Keys to Regain Control

Magic SysRq keys provide emergency controls if the system is deadlocked but keyboard input still works. Useful sequences include:

ALT+SysRq+E – Send SIGTERM to all processes, signaling shutdown
ALT+SysRq+I – Send SIGKILL to all processes except init, forcing reboot
ALT+SysRq+S – Attempt to sync all mounted filesystems safely before reboot
ALT+SysRq+U – Attempt to remount filesystems read-only before reboot

The goal is to regain control by eliminating processes and remounting filesystems cleanly.

Example Code for Triggering a Soft Reboot

The kernel_restart function provides a last resort to reboot the system if deadlocked:

#include 

void kernel_restart(char *cmd) {

  /* Attempt clean sync/remount of filesystems */
  emergency_sync();
  emergency_remount();

  /* Send SIGKILL to all processes except init */
  for_each_process(send_sig_kill);
   
  /* Reboot system */
  machine_restart(cmd);

}

Invoking kernel_restart() eliminates all processes and reboots as a last resort if deadlocked.

Forced Rebooting as a Last Resort

If the magic SysRq restart command does not work, forced rebooting may be required. This risks filesystem damage but may be necessary. Options include:

Pressing the hardware reset button to cold reboot
Power cycling the machine if no reset button exists
Using remote management cards to force reboot

These methods should only be used as a last resort since they risk filesystem corruption or data loss.

Checking Logs to Identify the Deadlock Cause

After rebooting, check system logs to identify the underlying cause:

# Check kernel ring buffer for lockdep info
dmesg | grep -i lockdep

# Check messages for stack traces with locking info  
journalctl -k | grep blocked

Analyzing these logs will provide details like inconsistent lock handling that is leading to the deadlocks. This info can guide code changes to prevent recurrences.

Additional Strategies for Resilience

To further improve kernel deadlock resilience:

Enable lockdep locking dependency runtime checking
Configure watchdog timers to reboot gracefully
Partition/scale resources to reduce contention probability

Enabling Lockdep for Runtime Deadlock Detection

The lockdep facility detects inconsistent lock handling at runtime. To enable:

Configure kernel with lockdep support enabled
Boot kernel with “lockdep.max_lock_depth=NN” parameter
Load supporting lockdep modules
Monitor dmesg and logs for lockdep reports

Lockdep provides early warning of inconsistent locks that lead to deadlocks.

Configuring the Watchdog to Automatically Reboot

A watchdog timer can automatically reboot the system if deadlocked. To enable:

Specify “softlockup_panic=1” kernel parameter to enable watchdog
Use /proc/sys/kernel/nmi_watchdog to configure watchdog timeout threshold
Timer automatically panics kernel if deadlocked past threshold

The watchdog provides hands-off recovery in case all else fails.

Scaling Resources to Reduce Contention

Contention for limited resources leads to deadlocks. Solutions include:

Vertical scaling – increase per-system resources like RAM, CPU, disk
Horizontal scaling – distribute load across systems
Partition resources into independent subsets to limit contention domains

Preventing resource exhaustion minimizes deadlock probability.