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Synchronization

September 15, 2006

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Outline

• Historical solutions• Synchronization Primitives• Brief word on concurrency• Semaphores in depth

– semid_ds– sem– Working with semaphores

• References

Peterson's Algorithm

boolean lockA = lockB = false;int turn = A; // or B

//Thread A:

lockA = true;turn = B;while( lockB && turn != A ){}

lockA = false;

//Thread B:

lockB = true;turn = A;while( lockA && turn != B ){}

lockB = false;

Dekker's Algorithm

boolean lockA = lockB = false;int turn = A; // or B

//Thread A:

lockA = true;

while(lockB)

{

if (turn != A)

{

lockA = false;

while(turn != A){}

lockA = true;

}

} lockA = false;

turn = B;

//Thread B:

lockB = true;

while(lockA)

{

if (turn != B)

{

lockB = false;

while(turn != B){}

lockB = true;

}

} lockB = false;

turn = A;

Synchronization Primitives

• Per-CPU variables• Atomic operation• Memory barrier• Spin lock• Seqlocks• Read-copy-update• Semaphore• Local interrupt disabling• Local softirq disabling

Synchronization Primitives – cont.

• Per-CPU Variables– Array of data structures that can be accessed by only 1 CPU– Only useful when data can be split across CPUs– Fails if kernel preemption is enabled (race conditions)

• Atomic operation– Read-modify-write

o atomic_read(v)o atomic_set(v, i) – (test & set)o atomic_add(i, v)

– Another set operates on bit masks

Synchronization Primitives – cont.

• Memory barrier– Avoids code optimization problems by creating a barrier that

ensures that all code before the barrier gets executed before code after the barrier

• Spin locks– Busy wait while blocked– Useful for multi-processor systems when the critical code

will execute very fasto 1 = unlocked, 0 = locked

– Read/Write Spin Locks

Synchronization Primitives – cont.

• Seqlocks– Read/Write spin locks that give high priority to writers

• Read-Copy Update– Allows many readers and writers– No locks or counters– Writer just copies the old data, updates it and changes the

pointer– Old data cannot be freed until all readers are finished

Synchronization Primitives – cont.

• Semaphores– Kernel and System V IPC (User)– count

o If count > 0 the resource is freeo count = 0 means the resource is 100% utilized but the wait

queue is emptyo If count < 0 the resource is 100% utilized and there are

processes in the wait queue

– wait – pointer to the wait queue– sleepers – flag that is set if there are processes that are

waiting blocked– More to come!

Synchronization Primitives – cont.

• Local Interrupt Disabling– Creates a critical section in the kernel so that even

hardware interrupts won’t break the execution of a set of statements

– CPU specific

• Local softirq disabling– Disable deferrable functions w/o disabling interrupts– Increment the softirq counter to disable; decrement the

softirq counter to enable

++ Concurrency

• Maximize the number of I/O devices operating– Disable interrupts for very short periods

• Maximize the number of productive CPUs– Avoid using spin locks if possible

More on Semaphores (semid_ds)

/* One semid data structure for each set of semaphores in the system. */ struct semid_ds

{ struct ipc_perm sem_perm; /*

permissions .. see ipc.h */ time_t sem_otime; /* last semop

time */ time_t sem_ctime; /* last change

time */ struct sem *sem_base; /* ptr to

first semaphore in array */ struct wait_queue *eventn; struct wait_queue *eventz; struct sem_undo *undo; /* undo

requests on this array */ ushort sem_nsems; /* no. of

semaphores in array */ };

•sem_perm – This is an instance of the ipc_perm

structure, which is defined for us in linux/ipc.h. This holds the permission information for the semaphore set, including the access permissions, and information about the creator of the set (uid, etc).

•sem_otime – Time of the last semop() operation

•sem_ctime – Time of the last change to this

structure (mode change, etc)

•sem_base – Pointer to the first semaphore in the

array (see next structure)

•sem_undo – Number of undo requests in this array

•sem_nsems – Number of semaphores in the

semaphore set (the array)

More on Semaphores (sem)

•/* One semaphore structure for each semaphore in the system. */

struct sem {

short sempid; /* pid of last operation */

ushort semval; /* current value */

ushort semncnt; /* num procs awaiting increase in semval */

ushort semzcnt; /* num procs awaiting semval = 0 */

};

•sem_pid – The PID (process ID) that performed

the last operation

•sem_semval – The current value of the semaphore

•sem_semncnt – Number of processes waiting for

resources to become available

•sem_semzcnt – Number of processes waiting for

100% resource utilization

Working with Semaphores

int semget ( key_t key, int nsems, int semflg );

int semop ( int semid, struct sembuf *sops, unsigned nsops);

struct sembuf

{

ushort sem_num; /* semaphore index in array */

short sem_op; /* semaphore operation */

short sem_flg; /* operation flags */ };

Working with Semaphores – cont.

int semctl ( int semid, int semnum, int cmd, union semun arg );

union semun {

int val; /* value for SETVAL */

struct semid_ds *buf; /* buffer for IPC_STAT & IPC_SET */

ushort *array; /* array for GETALL & SETALL */

struct seminfo *__buf; /* buffer for IPC_INFO */ void *__pad;

};

Semaphore Commands

•IPC_STAT – Retrieves the semid_ds structure for a

set, and stores it in the address of the buf argument in the semun union.

•IPC_SET – Sets the value of the ipc_perm

member of the semid_ds structure for a set. Takes the values from the buf argument of the semun union.

•IPC_RMID – Removes the set from the kernel.

•GETALL – Used to obtain the values of all

semaphores in a set. The integer values are stored in an array of unsigned short integers pointed to by the array member of the union.

•GETNCNT – Returns the number of processes

currently waiting for resources.

•GETPID – Returns the PID of the process which

performed the last semop call.

•GETVAL – Returns the value of a single

semaphore within the set.

•GETZCNT – Returns the number of processes

currently waiting for 100% resource utilization.

•SETALL – Sets all semaphore values with a set

to the matching values contained in the array member of the union.

•SETVAL – Sets the value of an individual

semaphore within the set to the val member of the union.

References

• D. Bovet and M. Cesati. Understanding the Linux Kernel, Third Edition.O'Reilly and Associates, Inc., 2002.

• The Linux Documentation Project: http://www.tldp.org/LDP/lpg/node46.html

• R. Kline. Peterson's Algorithm, 2006. http://www.cs.wcupa.edu/~rkline/OS/Peterson.html