03-03-2009
POSIX threads and data safety
I created multiple POSIX threads (on readhat Linux) in a C program in my app. What I am doing is - I am creating threads equal to the number of CPUs in the system and and equal number of instances of a certain data structure, basically a queue implementation. I am assigning one ID to the thread and same ID is given to the queue. Threads process data from the queue. Thread #1 always process data from queue #1 (and from this queue only) and so on. However, data that is processed from queue #1 has some state dependency wrt the previously processed data from the queue. Think of a network traffic analyser - to establish a connection, we need to take previous state into consideration.
My question is -
(a) in such a scenario, will my app be (thread) safe ?
(b) Yes, data from a queue is picked by a corresponding thread but what if the threads switch CPUs ?
(c) Do posix threads switch CPUs ?
Last edited by radiatejava; 03-03-2009 at 04:31 AM..
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LEARN ABOUT HPUX
runqueue
RUNQUEUE(9) BSD Kernel Developer's Manual RUNQUEUE(9)
NAME
chooseproc, procrunnable, remrunqueue, setrunqueue -- manage the queue of runnable processes
SYNOPSIS
#include <sys/param.h>
#include <sys/proc.h>
extern struct rq itqueues[];
extern struct rq rtqueues[];
extern struct rq queues[];
extern struct rq idqueues[];
struct thread *
choosethread(void);
int
procrunnable(void);
void
remrunqueue(struct thread *td);
void
setrunqueue(struct thread *td);
DESCRIPTION
The run queue consists of four priority queues: itqueues for interrupt threads, rtqueues for realtime priority processes, queues for time
sharing processes, and idqueues for idle priority processes. Each priority queue consists of an array of NQS queue header structures. Each
queue header identifies a list of runnable processes of equal priority. Each queue also has a single word that contains a bit mask identify-
ing non-empty queues to assist in selecting a process quickly. These are named itqueuebits, rtqueuebits, queuebits, and idqueuebits. The
run queues are protected by the sched_lock mutex.
procrunnable() returns zero if there are no runnable processes other than the idle process. If there is at least one runnable process other
than the idle process, it will return a non-zero value. Note that the sched_lock mutex does not need to be held when this function is
called. There is a small race window where one CPU may place a process on the run queue when there are currently no other runnable processes
while another CPU is calling this function. In that case the second CPU will simply travel through the idle loop one additional time before
noticing that there is a runnable process. This works because idle CPUs are not halted in SMP systems. If idle CPUs are halted in SMP sys-
tems, then this race condition might have more serious repercussions in the losing case, and procrunnable() may have to require that the
sched_lock mutex be acquired.
choosethread() returns the highest priority runnable thread. If there are no runnable threads, then the idle thread is returned. This func-
tion is called by cpu_switch() and cpu_throw() to determine which thread to switch to. choosethread() must be called with the sched_lock
mutex held.
setrunqueue() adds the thread td to the tail of the appropriate queue in the proper priority queue. The thread must be runnable, i.e. p_stat
must be set to SRUN. This function must be called with the sched_lock mutex held.
remrunqueue() removes thread td from its run queue. If td is not on a run queue, then the kernel will panic(9). This function must be
called with the sched_lock mutex held.
SEE ALSO
cpu_switch(9), scheduler(9), sleepqueue(9)
BSD
November 3, 2000 BSD