Analysis of Kernel Locking Mechanisms

1

• Analysis of Kernel Locking Mechanisms
• for Linux Kernel Programming
• Dr. B. Thangaraju
• Manager Talent Transformation
• Wipro Technologies, Bangalore.

2
Locking Mechanisms

• Kernel Control Path
• Global Data
• Shared Resources
• Non-preemptive Kernel
• Disable Interrupts
• Atomic Operations
• Locking Mechanism

3
Choices of Lock

• Semaphore
• Spin Lock
• Integer and Bit Operations
• Wait Queue
• Capabilities and Restricted
• 
  Operations

4
Semaphore

• Synchronization Tool
• An Integer Number
• P ( ) And V ( ) Operators
• Busy Waiting
• Types of Semaphore
• Implementation
• Expensive

5
Semaphore

• include ltasm/semaphore.hgt
• struct semaphore atomic_t count int sleepers
• wait_queue_head_t wait
• void sema_init (struct semaphore sem, int value
  )
• int down (struct semaphore sem)
• int down_interruptible (struct semaphore sem)
• void up (struct semaphore sem)

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Spin Lock

• Little Locking Overhead
• Ideal for Small Critical Section
• Never Put a Process to Sleep
• Spin in Busy-Wait Loop
• include ltlinux/spinlock.hgt
• typedef struct
• volatile unsigned int lock
• spinlock_t

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Spin Lock- Functions

• spin_lock_init ( spinlock_t lock)
• spin_lock (spinlock_t lock)
• spin_unlock ( spinlock_t lock)
• spin_is_locked ( spinlock_t lock)
• spin_trylock ( spinlock_t lock)
• spin_lock_irqsave ( spinlock_t lock, unsigned
  long flags)
• spin_lock_irqrestore ( spinlock_t lock, unsig
  long flags )
• spin_lock_irq (/bh) (spinlock_t lock)
• spin_unlock_irq (/bh) (spinlock_t lock)

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Spin Lock reader and writer

• rwlock_t my_lock RW_LOCK_UNLOCKED
• read_lock (rwlock_t lock)
• read_lock_irqsave (rwlock_t lock, unsigned long
  flags)
• read_lock_irq (rwlock_t lock)
• read_lock_bh (rwlock_t lock)
• write_lock (rwlock_t lock)
• write_lock_irqsave (rwlock_t lock, unsigned long
  flags)
• write_lock_irq (rwlock_t lock)
• write_lock_bh (rwlock_t lock)

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Atomic Lock

• Guaranteed to be an atomic even on SMP
• Operate on a bit or an integer
• Assembler or C functions dep. on arch
• typedef volatile int counter
• atomic_t
• include ltasm/bitops.hgt
• include ltasm/atomic.hgt

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Atomic Integer Operations

• void atomic_add (atomic_t i, atomic_t v)
• void atomic_sub (atomic_t i, atomic_t v)
• int atomic_sub_and_test (atomic_t i, atomic_t
  v)
• void atomic_inc (atomic_t v)
• void atomic_dec (atomic_t v)
• int atomic_dec_and_test (atomic_t v)

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Atomic bit Operations

• void set_bit (int nr, void addr)
• void clear_bit (int nr, void addr)
• void change_bit (int nr, void addr)
• test_bit (int nr, void addr)
• int test_and_set_bit (int nr, void addr)
• int test_and_clear_bit (int nr, void addr)
• int test_and_change_bit (int nr, void addr)

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Wait Queues

• A queue of a processes
• waiting for an event.
• Used in
• page allocator
• kswapd kernel daemon
• device drivers.
• Many ways to ask a process to wait
• Many ways to wakeup a process

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Wait Queues - Functions

• wait_queue_head_t my_queue //
  declaration
• init_waitqueue_head (my_queue) //
  initialization
• sleep_on (wait_queue_head_t queue)
• interruptible_sleep_on (wait_queue_head_t
  queue)
• sleep_on_timeout (wait_queue_head_t queue, long
  
• 
  timeout)
• interruptible_sleep_on_timeout (wait_queue_head_t
  
• 
  queue, long timeout)
• void wait_event (wait_queue_head_t queue, int
  condition)
• int wait_event_interruptible (wait_queue_head_t
  queue,
• 
  int condition)

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Wait Queues - Functions

• Ways of Wake Up
• wake_up (wait_queue_head_t queue)
• wake_up_interruptible (wait_queue_head_t queue)
• wake_up_sync (wait_queue_head_t queue)
• wake_up_interruptible_sync (wait_queue_head_t
  queue)

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Restricted Operations

• Restricted for certain operations by capabilities
• include ltlinux/capability.hgt
• CAP_DAC_OVERRIDE
• CAP_NET_ADMIN
• CAP_SYS_MODULE
• CAP_SYS_RAWIO
• CAP_SYS_ADMIN
• CAP_SYS_TTY_CONFIG

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Summary

• Locking Mechanisms
• Semaphore
• Spin Lock
• Atomic Integer Operations
• Bit Operations
• Wait Queues
• Capabilities and Restricted Operations

17

• Thank You