Download Synchronization

CSC 660: Advanced OS
Synchronization
CSC 660: Advanced Operating Systems
Slide #1
Topics
1.
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3.
4.
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6.
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8.
Race Conditions
Critical Sections
Kernel Concurrency
What needs Protection?
Atomic Operations
Spin Locks
Semaphores
Deadlock
CSC 660: Advanced Operating Systems
Slide #2
Race Conditions
Process A
4
abc.pdf
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prog.c
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as2.txt
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out = 4
in = 7
Process B
CSC 660: Advanced Operating Systems
Slide #3
Race Conditions
1. Process A read in, stores in local var slot.
2. Timer interrupt.
3. Scheduler switches to process B.
4. Process B reads in, stores in local var slot.
5. Process B stores filename in slot 7 (slot).
6. Process B updates in to be 8.
7. Scheduler eventually switches to process A.
8. Process A writes filename in slot 7 (slot).
9. Process A computes in = slot + 1.
10. Process A updates in to be 8.
CSC 660: Advanced Operating Systems
Slide #4
Critical Sections
• How can we prevent race conditions?
– Prohibit multiple processes from accessing
shared resource at the same time.
– Critical section is code that accesses shared
resource.
CSC 660: Advanced Operating Systems
Slide #5
Critical Sections
1. No two processes may be simultaneously
inside their critical sections.
2. No assumptions may be made about speed
or number of CPUs.
3. No process running outside its critical
section may block other processes.
4. No process should have to wait forever to
enter its critical section.
CSC 660: Advanced Operating Systems
Slide #6
Critical Sections
CSC 660: Advanced Operating Systems
Slide #7
Kernel Concurrency
1.
2.
3.
4.
5.
Interrupts
Softirqs and tasklets
Kernel preemption
Sleeping
SMP
CSC 660: Advanced Operating Systems
Slide #8
What needs Protection?
• What needs protection?
– Global kernel data structures.
– Shared data between process/interrupt context.
– Shared data between interrupt handlers.
• What doesn’t?
–
–
–
–
Data local to executing thread (i.e., stack.)
Local variables.
Dynamically allocated data w/ only local ptrs.
Per-CPU data doesn’t need SMP protection.
CSC 660: Advanced Operating Systems
Slide #9
Why do we need atomicity?
Problem: Two processes incrementing i.
Uniprocessor Version
A: read i(7)
A: incr i(7 -> 8)
B: read i(8)
B: incr i(8 -> 9)
Process A
read i(7)
incr i(7 -> 8)
write i(8)
CSC 660: Advanced Operating Systems
Process B
read i(7)
incr i(7 -> 8)
write i(8)
Slide #10
Atomic Operations
Atomic operations are indivisible.
Process A
atomic_inc i (7->8)
Process B
atomic_inc i (8->9)
Provided by atomic_t in the kernel.
Operations: atomic_{read,set,add,sub,inc,dec,dec_and_test)
x86 assembly: lock byte preceding opcode makes atomic.
CSC 660: Advanced Operating Systems
Slide #11
Atomicity doesn’t provide Ordering
One atomic order of operations:
Process A
atomic_inc i (7->8)
Process B
atomic_inc i (8->9)
Another atomic order of operations:
Process A
Process B
atomic_inc i (8->9)
atomic_inc i (7->8)
CSC 660: Advanced Operating Systems
Slide #12
Barriers provide Ordering
• Optimization Barriers
– Prevent compiler from re-ordering instructions.
– Compiler doesn’t know when interrupts or other
processors may read/write your data.
– Kernel provides barrier() macro.
• Memory Barriers
– Read/write barriers prevent loads/stores from
being re-ordered across barrier.
– Kernel provides rmb(), wmb() macros.
CSC 660: Advanced Operating Systems
Slide #13
Spin Locks
• If lock “open”
– Sets lock bit with atomic test-and-set.
– Continues into critical region.
• else lock “closed”
– Code “spins” in busy wait loop until available.
– Kernel-preemption or SMP will interrupt.
CSC 660: Advanced Operating Systems
Slide #14
Spin Lock Functions
spin_lock_init(spinlock_t *lock)
Initialize spin lock to 1 (unlocked).
spin_lock(spinlock_t *lock)
Spin until lock becomes 1, then set to 0 (locked).
spin_lock_irqsave(spinlock_t *l, u flags)
Like spin_lock() but disables and saves interrupts.
Always use an IRQ disabling variant in interrupt context.
spin_unlock(spinlock_t *lock)
Set spin lock to 1 (unlocked).
spin_lock_irqrestore(spinlock_t *l, u flags)
Like spin_lock(), but restores interrupt status.
spin_trylock(spinlock_t *lock)
Set lock to 0 if unlocked and return 1; return 0 if locked.
CSC 660: Advanced Operating Systems
Slide #15
Spin Lock
1. Disables kernel pre-emption.
2. Atomic test-and-sets lock.
3. If old value positive
Lock acquired.
4. Else
Enables pre-emption.
If break_lock is 0, sets to 1 to indicate a task is waiting.
Busy wait loop
while (spin_is_locked(lock))
cpu_relax(); # pause instruction on P4
Goto 1.
CSC 660: Advanced Operating Systems
Slide #16
Semaphores
Integer value with atomic access.
If S>0, semaphore prevents access.
Using a semaphore for mutual exclusion:
down(S);
/* critical section */
up(S);
CSC 660: Advanced Operating Systems
Slide #17
Semaphores
Down (P): Request to enter critical region.
If S > 0, decrements S, enters region.
Else process sleeps until semaphore is released.
Up (V): Request to exit critical region.
Increments S.
If S > 0, wakes sleeping processes.
CSC 660: Advanced Operating Systems
Slide #18
Linux Semaphores
DECLARE_MUTEX(sem);
Static declares a mutex semaphore.
void init_MUTEX(struct semaphore *sem);
Dynamic declaration of a mutex semaphore.
void down(struct semaphore *sem);
Decrements semaphore and sleeps.
int down_interruptible(struct semaphore *sem);
Same as down() but returns on user interrupt.
int down_trylock(struct semaphore *sem);
Same as down() but returns immediately if not available.
void up(struct semaphore *sem);
Releases semaphore.
CSC 660: Advanced Operating Systems
Slide #19
Linux Semaphores
#include <asm/semaphore.h>
struct semaphore sem;
init_MUTEX(&sem);
if (down_interruptible(&sem))
return –ERESTARTSYS; /* user interrupt */
/*
* critical section
*/
up(&sem);
CSC 660: Advanced Operating Systems
Slide #20
Tradeoffs
Spin Locks
Busy waits waste CPU cycles.
Semaphores
Context switch on sleep is expensive.
CSC 660: Advanced Operating Systems
Slide #21
Deadlocks
Single process deadlock:
A: spin_lock(A)
A: spin_lock(A)
Two process deadlock:
Process A
Process B
down(A)
down(B)
down(B)
down(A)
CSC 660: Advanced Operating Systems
Slide #22
Deadlock Prevention
Lock Ordering
If you acquire multiple locks, you must always
acquire them in the same order.
Don’t double acquire a lock.
Always use interrupt-disabling variants of
spin_lock() in interrupt context.
Always release locks.
Be sure that your code will always make
appropriate calls to release locks.
CSC 660: Advanced Operating Systems
Slide #23
Key Points
• Synchronization is essential for accessing shared
kernel data structures.
• Kernel provides two major types:
– Spin Locks
– Semaphores
– and many variants.
• System performance depends strongly on the type
of synchronization used.
• Synchronization must be used carefully and
consistently to avoid deadlocks.
CSC 660: Advanced Operating Systems
Slide #24
References
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6.
Daniel P. Bovet and Marco Cesati, Understanding the
Linux Kernel, 3rd edition, O’Reilly, 2005.
Johnathan Corbet et. al., Linux Device Drivers, 3rd
edition, O’Reilly, 2005.
Robert Love, Linux Kernel Development, 2nd edition,
Prentice-Hall, 2005.
Claudia Rodriguez et al, The Linux Kernel Primer,
Prentice-Hall, 2005.
Peter Salzman et. al., Linux Kernel Module Programming
Guide, version 2.6.1, 2005.
Andrew S. Tanenbaum, Modern Operating Systems, 3rd
edition, Prentice-Hall, 2005.
CSC 660: Advanced Operating Systems
Slide #25