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pthread_mutex_lock in ANSI C vs using Atomic builtins of GCC

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# 1  
Old 08-25-2010
pthread_mutex_lock in ANSI C vs using Atomic builtins of GCC
I have a program which has 7-8 threads, and lots of shared variables; these variables (and also they may not the primitive type, they may be enum or struct ), then they may read/write by different threads at the same time.

Now, my design is like this,

Code:
typedef unsigned short int UINT16;
struct STRUCT {
  UINT16 uint16;
  int INT;
}
enum ENUM = { ... };

/* Shared Variables. */
struct STRUCT Shared_VariableA;
enum ENUM Shared_VariableB;

UINT16 GetVA () {
    pthread_mutex_lock( &_L_LOCK );
        UINT16 TMP = Shared_VariableA.uint16;
    pthread_mutex_lock( &_L_LOCK );
    return TMP;
}

void SetVA ( UINT16 in ) {
     pthread_mutex_lock( &_L_LOCK );
         Shared_VariableA.uint16 = in;
     pthread_mutex_lock( &_L_LOCK );
 }

Is it enough to guarantee that only a thread can write/read shared variables at the same time?

How about the following method? Same?
From the reference : Techie Stuff Atomic Operations

Code:
/* Initialization. */
atomic_t uint16 = ATOMIC_INIT ( 0 );

UINT16 GetVA () {
         UINT16 TMP = atomic_read( &uint16 );  /* Force convert 'int' to UINT16 since atomic_read() returns an int (32 bits). */
        return TMP;
 }
 
 void SetVA ( UINT16 in ) {
       atomic_set( &uint16,in );
  }

Whether getVA() will return an arbitrary value when SetVA() is running by other threads?
# 2  
Old 08-25-2010
Yes, the pthread solution more than sufficiently protects it. (A pthread_rwlock would let readers operate at the same time and only block them for writes.)

If the gcc atomic operations are truly atomic, getva shouldn't ever return garbage.
# 3  
Old 08-26-2010
I assume that this is a typo - the second pthread_mutex_lock() should be a pthread_mutex_unlock().
Code:
     pthread_mutex_lock( &_L_LOCK );
         Shared_VariableA.uint16 = in;
     pthread_mutex_lock( &_L_LOCK );

This User Gave Thanks to fpmurphy For This Post:
sehang
# 4  
Old 08-26-2010
Quote:
Originally Posted by Corona688
Yes, the pthread solution more than sufficiently protects it. (A pthread_rwlock would let readers operate at the same time and only block them for writes.)

If the gcc atomic operations are truly atomic, getva shouldn't ever return garbage.
Code:
/* Shared Variables. */
struct STRUCT Shared_VariableA;
enum ENUM Shared_VariableB;

UINT16 GetVA () {
    pthread_mutex_lock( &_L_LOCK );
        UINT16 TMP = Shared_VariableA.uint16;
    pthread_mutex_unlock( &_L_LOCK );
    return TMP;
}

I have a question, if a shared variable may read/write by some threads and with pthread_mutex_lock(), is the variable requires a 'volatile'?
e.g.
Code:
volatile struct STRUCT Shared_VariableA;



---------- Post updated at 01:03 PM ---------- Previous update was at 01:02 PM ----------

I am sorry, it should be:
Code:
     pthread_mutex_lock( &_L_LOCK );
         Shared_VariableA.uint16 = in;
     pthread_mutex_unlock( &_L_LOCK );

Quote:
Originally Posted by fpmurphy
I assume that this is a typo - the second pthread_mutex_lock() should be a pthread_mutex_unlock().
Code:
     pthread_mutex_lock( &_L_LOCK );
         Shared_VariableA.uint16 = in;
     pthread_mutex_lock( &_L_LOCK );

# 5  
Old 08-26-2010
If properly bounded by mutexes(or other synchronization calls) it shouldn't need "volatile". Atomic operations might still need it.
This User Gave Thanks to Corona688 For This Post:
sehang
# 6  
Old 08-26-2010
It is possible create a variable with "volatile" but it is not a primitive type, e.g.

Code:
struct STRUCT {
  volatile int S1;
  volatile char S2;
  volatile  unsigned short int S3;
}
volatile struct STRUCT v1;
volatile enum ENUM v2;
volatile char V3;
volatile unsigned int V4;

v1->S1 = 1;
v1->S2 = 2;
v1->S3 = 3;

v2 = 4;

v3 = 5;

v4 = 6;

Are above operations atomic?
Is any case that "volatile" can not guarantee atomic operation?
# 7  
Old 08-26-2010
"volatile" has nothing to do with atomic, "volatile" just tells the compiler to never assume that variable is whatever it set it to last. (otherwise, it might assume it never changes and hardcode it in the instructions.)

No variables (except the gcc atomic extensions) are guaranteed to be atomic, at all, ever. There's other problems than being atomic, too -- on a multicore system, one core might have a different copy of the memory still in cache, causing even "volatile" to fail. You need to inform other cores you're modifying values out from under them, which is what memory barriers are for. pthreads does memory barriers for you. I don't know if gcc atomic ops do.
Last edited by Corona688; 08-26-2010 at 05:33 PM..
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