[patch][commit] Single Mutex for sidutil::blocking_component

[Dave Brolley <brolley@redhat.com>]

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Hi,

I've committed the attached patch which simplifies the implementation of 
sidutil::blocking_component. The use of two mutex/condition variable 
pairs for synchronizing the parent and child threads was unnecessary 
and, if you look at the existing code, you will see that they were 
always locked/unlocked/blocked in unison. The implementation now uses a 
single mutex/condition variable pair.

The patch also adds checking that a return from pthread_cond_wait was 
not spontaneous and makes the control_status member volatile for this 
purpose.

Tested extensively using the MeP port which is the only committed port 
using this class.

Dave

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2007-02-19  Dave Brolley  <brolley@redhat.com>

	* sidblockingutil.h (sidutil::blocking_component): Remove
	child_resume_mutex, child_stopped_mutex, chiuld_resume_condition and
	child_stopped_condition. Use a single mutex/condition pair:
	child_sync_mutex and child_sync_condition. Handle spontaneous returns
	from pthread_cond_wait.
	(control_status): Now volatile.


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Index: sid/include/sidblockingutil.h
===================================================================
RCS file: /cvs/src/src/sid/include/sidblockingutil.h,v
retrieving revision 1.3
diff -c -p -r1.3 sidblockingutil.h
*** sid/include/sidblockingutil.h	5 Feb 2007 20:28:42 -0000	1.3
--- sid/include/sidblockingutil.h	19 Feb 2007 16:26:33 -0000
*************** namespace sidutil
*** 49,57 ****
        {
  	log (10, "%s: child_init\n", name.c_str ());
  	assert (child_created);
! 	// Lock both mutexes
! 	pthread_mutex_lock (& child_resume_mutex);
! 	pthread_mutex_lock (& child_stopped_mutex);
        }
  
    protected:
--- 49,56 ----
        {
  	log (10, "%s: child_init\n", name.c_str ());
  	assert (child_created);
! 	// Lock the syncronization mutex
! 	pthread_mutex_lock (& child_sync_mutex);
        }
  
    protected:
*************** namespace sidutil
*** 59,73 ****
        {
  	log (10, "%s: parent_init\n", name.c_str ());
  
! 	// Create mutexes for synchronizing the parent and child threads
! 	pthread_mutex_init (& child_resume_mutex, NULL);
! 	pthread_cond_init (& child_resume_condition, NULL);
! 	pthread_mutex_init (& child_stopped_mutex, NULL);
! 	pthread_cond_init (& child_stopped_condition, NULL);
! 
! 	// Lock both mutexes
! 	pthread_mutex_lock (& child_resume_mutex);
! 	pthread_mutex_lock (& child_stopped_mutex);
  	control_status = ctl_parent;
        }
  
--- 58,69 ----
        {
  	log (10, "%s: parent_init\n", name.c_str ());
  
! 	// Create a mutex for synchronizing the parent and child threads
! 	pthread_mutex_init (& child_sync_mutex, NULL);
! 	pthread_cond_init (& child_sync_condition, NULL);
! 
! 	// Lock the mutex
! 	pthread_mutex_lock (& child_sync_mutex);
  	control_status = ctl_parent;
        }
  
*************** namespace sidutil
*** 121,139 ****
  	// Signal the child to resume
  	assert (control_status != ctl_child_start);
  	control_status = ctl_child_start;
! 	pthread_cond_signal (& child_resume_condition);
! 
! 	// Unlock the mutex so that the child can gain control
! 	pthread_mutex_unlock (& child_resume_mutex);
  
  	// Wait for the return signal from the child
! 	pthread_cond_wait (& child_stopped_condition, & child_stopped_mutex);
! 
! 	// Reacquire the mutex so that the child can gain control
! 	pthread_mutex_lock (& child_resume_mutex);
  
- 	// Check the value of control_status
- 	assert (control_status != ctl_child_start);
  	return control_status;
        }
  
--- 117,132 ----
  	// Signal the child to resume
  	assert (control_status != ctl_child_start);
  	control_status = ctl_child_start;
! 	log (11, "%s: parent signalling the child thread\n", name.c_str ());
! 	pthread_cond_signal (& child_sync_condition);
  
  	// Wait for the return signal from the child
! 	do {
! 	  log (11, "%s: parent waiting for child thread\n", name.c_str ());
! 	  pthread_cond_wait (& child_sync_condition, & child_sync_mutex);
! 	} while (control_status == ctl_child_start); 
! 	log (11, "%s: parent regains control\n", name.c_str ());
  
  	return control_status;
        }
  
*************** namespace sidutil
*** 168,185 ****
  
  	// Signal the parent that we're stopped
  	log (11, "%s: child signalling the parent thread\n", name.c_str ());
! 	pthread_cond_signal (& child_stopped_condition);
! 
! 	// Unlock the mutex so that the parent can gain control
! 	pthread_mutex_unlock (& child_stopped_mutex);
  
  	// Wait for a signal to resume
! 	log (11, "%s: child waiting for parent thread\n", name.c_str ());
! 	pthread_cond_wait (& child_resume_condition, & child_resume_mutex);
! 
! 	// Reacquire the stopped mutex
! 	pthread_mutex_lock (& child_stopped_mutex);
! 	assert (control_status == ctl_child_start);
        }
  
      void set_blockable ()
--- 161,174 ----
  
  	// Signal the parent that we're stopped
  	log (11, "%s: child signalling the parent thread\n", name.c_str ());
! 	pthread_cond_signal (& child_sync_condition);
  
  	// Wait for a signal to resume
! 	do {
! 	  log (11, "%s: child waiting for parent thread\n", name.c_str ());
! 	  pthread_cond_wait (& child_sync_condition, & child_sync_mutex);
! 	} while (control_status != ctl_child_start);
! 	log (11, "%s: child regains control\n", name.c_str ());
        }
  
      void set_blockable ()
*************** namespace sidutil
*** 198,208 ****
      bool child_created;
      pthread_t child_thread;
      void *(*child_thread_function)(void *);
!     pthread_mutex_t child_resume_mutex;
!     pthread_cond_t child_resume_condition;
!     pthread_mutex_t child_stopped_mutex;
!     pthread_cond_t child_stopped_condition;
!     enum
        {
  	ctl_parent, ctl_child_start, ctl_child_blocked, ctl_child_complete
        } control_status;
--- 187,195 ----
      bool child_created;
      pthread_t child_thread;
      void *(*child_thread_function)(void *);
!     pthread_mutex_t child_sync_mutex;
!     pthread_cond_t child_sync_condition;
!     volatile enum
        {
  	ctl_parent, ctl_child_start, ctl_child_blocked, ctl_child_complete
        } control_status;