In the words of the author:

  o The polling mechanism for I/O readiness was changed from
    select() to poll().  In additon, a wrapped version of poll()
    is now provided.

  o The wrapped select routine now converts each fd_set to a
    poll array so that the thread scheduler doesn't have to
    perform a bitwise search for selected fds each time file
    descriptors are polled for I/O readiness.

  o The thread scheduler was modified to use a new queue (_workq)
    for threads that need work.  Threads waiting for I/O readiness
    and spinblocks are added to the work queue in addition to the
    waiting queue.  This reduces the time spent forming/searching
    the array of file descriptors being polled.

  o The waiting queue (_waitingq) is now maintained in order of
    thread wakeup time.  This allows the thread scheduler to
    find the nearest wakeup time by looking at the first thread
    in the queue instead of searching the entire queue.

  o Removed file descriptor locking for select/poll routines.  An
    application should not rely on the threads library for providing
    this locking; if necessary, the application should use mutexes
    to protect selecting/polling of file descriptors.

  o Retrieve and use the kernel clock rate/resolution at startup
    instead of hardcoding the clock resolution to 10 msec (tested
    with kernel running at 1000 HZ).

  o All queues have been changed to use queue.h macros.  These
    include the queues of all threads, dead threads, and threads
    waiting for file descriptor locks.

  o Added reinitialization of the GC mutex and condition variable
    after a fork.  Also prevented reallocation of the ready queue
    after a fork.

  o Prevented the wrapped close routine from closing the thread
    kernel pipes.

  o Initialized file descriptor table for stdio entries at thread
    init.

  o Provided additional flags to indicate to what queues threads
    belong.

  o Moved TAILQ initialization for statically allocated mutex and
    condition variables to after the spinlock.

  o Added dispatching of signals to pthread_kill.  Removing the
    dispatching of signals from thread activation broke sigsuspend
    when pthread_kill was used to send a signal to a thread.

  o Temporarily set the state of a thread to PS_SUSPENDED when it
    is first created and placed in the list of threads so that it
    will not be accidentally scheduled before becoming a member
    of one of the scheduling queues.

  o Change the signal handler to queue signals to the thread kernel
    pipe if the scheduling queues are protected.  When scheduling
    queues are unprotected, signals are then dequeued and handled.

  o Ensured that all installed signal handlers block the scheduling
    signal and that the scheduling signal handler blocks all
    other signals.  This ensures that the signal handler is only
    interruptible for and by non-scheduling signals.  An atomic
    lock is used to decide which instance of the signal handler
    will handle pending signals.

  o Removed _lock_thread_list and _unlock_thread_list as they are
    no longer used to protect the thread list.

  o Added missing RCS IDs to modified files.

  o Added checks for appropriate queue membership and activity when
    adding, removing, and searching the scheduling queues.  These
    checks add very little overhead and are enabled when compiled
    with _PTHREADS_INVARIANTS defined.  Suggested and implemented
    by Tor Egge with some modification by me.

  o Close a race condition in uthread_close.  (Tor Egge)

  o Protect the scheduling queues while modifying them in
    pthread_cond_signal and _thread_fd_unlock.  (Tor Egge)

  o Ensure that when a thread gets a mutex, the mutex is on that
    threads list of owned mutexes.  (Tor Egge)

  o Set the kernel-in-scheduler flag in _thread_kern_sched_state
    and _thread_kern_sched_state_unlock to prevent a scheduling
    signal from calling the scheduler again.  (Tor Egge)

  o Don't use TAILQ_FOREACH macro while searching the waiting
    queue for threads in a sigwait state, because a change of
    state destroys the TAILQ link.  It is actually safe to do
    so, though, because once a sigwaiting thread is found, the
    loop ends and the function returns.  (Tor Egge)

  o When dispatching signals to threads, make the thread inherit
    the signal deferral flag of the currently running thread.
    (Tor Egge)

Submitted by: Daniel Eischen <eischen@vigrid.com> and
              Tor Egge <Tor.Egge@fast.no>

1 files changed, 24 insertions, 87 deletions

diff --git a/lib/libpthread/thread/thr_gc.c b/lib/libpthread/thread/thr_gc.c
index 510c51f..e29e29b 100644
--- a/lib/libpthread/thread/thr_gc.c
+++ b/lib/libpthread/thread/thr_gc.c
@@ -29,7 +29,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- *	$Id: uthread_gc.c,v 1.2 1998/09/30 19:17:51 dt Exp $
+ *	$Id: uthread_gc.c,v 1.3 1999/03/23 05:07:55 jb Exp $
  *
  * Garbage collector thread. Frees memory allocated for dead threads.
  *
@@ -74,20 +74,26 @@ _thread_gc(pthread_addr_t arg)
 			/* Dump thread info to file. */
 			_thread_dump_info();
 
-		/* Lock the thread list: */
-		_lock_thread_list();
+		/*
+		 * Defer signals to protect the scheduling queues from
+		 * access by the signal handler:
+		 */
+		_thread_kern_sig_defer();
 
 		/* Check if this is the last running thread: */
-		if (_thread_link_list == _thread_run &&
-		    _thread_link_list->nxt == NULL)
+		if (TAILQ_FIRST(&_thread_list) == _thread_run &&
+		    TAILQ_NEXT(_thread_run, tle) == NULL)
 			/*
 			 * This is the last thread, so it can exit
 			 * now.
 			 */
 			f_done = 1;
 
-		/* Unlock the thread list: */
-		_unlock_thread_list();
+		/*
+		 * Undefer and handle pending signals, yielding if
+		 * necessary:
+		 */
+		_thread_kern_sig_undefer();
 
 		/*
 		 * Lock the garbage collector mutex which ensures that
@@ -100,48 +106,24 @@ _thread_gc(pthread_addr_t arg)
 		p_stack = NULL;
 		pthread_cln = NULL;
 
-		/* Point to the first dead thread (if there are any): */
-		pthread = _thread_dead;
-
-		/* There is no previous dead thread: */
-		pthread_prv = NULL;
-
 		/*
 		 * Enter a loop to search for the first dead thread that
 		 * has memory to free.
 		 */
-		while (p_stack == NULL && pthread_cln == NULL &&
-		    pthread != NULL) {
-			/* Save a pointer to the next thread: */
-			pthread_nxt = pthread->nxt_dead;
-
+		for (pthread = TAILQ_FIRST(&_dead_list);
+		    p_stack == NULL && pthread_cln == NULL && pthread != NULL;
+		    pthread = TAILQ_NEXT(pthread, dle)) {
 			/* Check if the initial thread: */
-			if (pthread == _thread_initial)
+			if (pthread == _thread_initial) {
 				/* Don't destroy the initial thread. */
-				pthread_prv = pthread;
-
+			}
 			/*
 			 * Check if this thread has detached:
 			 */
 			else if ((pthread->attr.flags &
 			    PTHREAD_DETACHED) != 0) {
-				/*
-				 * Check if there is no previous dead
-				 * thread:
-				 */
-				if (pthread_prv == NULL)
-					/*
-					 * The dead thread is at the head
-					 * of the list: 
-					 */
-					_thread_dead = pthread_nxt;
-				else
-					/*
-					 * The dead thread is not at the
-					 * head of the list: 
-					 */
-					pthread_prv->nxt_dead =
-					    pthread->nxt_dead;
+				/* Remove this thread from the dead list: */
+				TAILQ_REMOVE(&_dead_list, pthread, dle);
 
 				/*
 				 * Check if the stack was not specified by
@@ -162,14 +144,12 @@ _thread_gc(pthread_addr_t arg)
 				 * be freed outside the locks:
 				 */
 				pthread_cln = pthread;
+
 			} else {
 				/*
 				 * This thread has not detached, so do
-				 * not destroy it: 
-				 */
-				pthread_prv = pthread;
-
-				/*
+				 * not destroy it.
+				 *
 				 * Check if the stack was not specified by
 				 * the caller to pthread_create and has not
 				 * been destroyed yet: 
@@ -189,9 +169,6 @@ _thread_gc(pthread_addr_t arg)
 					pthread->stack = NULL;
 				}
 			}
-
-			/* Point to the next thread: */
-			pthread = pthread_nxt;
 		}
 
 		/*
@@ -229,52 +206,12 @@ _thread_gc(pthread_addr_t arg)
 		 */
 		if (p_stack != NULL)
 			free(p_stack);
-		if (pthread_cln != NULL) {
-			/* Lock the thread list: */
-			_lock_thread_list();
-
-			/*
-			 * Check if the thread is at the head of the
-			 * linked list.
-			 */
-			if (_thread_link_list == pthread_cln)
-				/* There is no previous thread: */
-				_thread_link_list = pthread_cln->nxt;
-			else {
-				/* Point to the first thread in the list: */
-				pthread = _thread_link_list;
-
-				/*
-				 * Enter a loop to find the thread in the
-				 * linked list before the thread that is
-				 * about to be freed.
-				 */
-				while (pthread != NULL &&
-				    pthread->nxt != pthread_cln)
-					/* Point to the next thread: */
-					pthread = pthread->nxt;
-
-				/* Check that a previous thread was found: */
-				if (pthread != NULL) {
-					/*
-					 * Point the previous thread to
-					 * the one after the thread being
-					 * freed: 
-					 */
-					pthread->nxt = pthread_cln->nxt;
-				}
-			}
-
-			/* Unlock the thread list: */
-			_unlock_thread_list();
-
+		if (pthread_cln != NULL)
 			/*
 			 * Free the memory allocated for the thread
 			 * structure.
 			 */
 			free(pthread_cln);
-		}
-		
 	}
 	return (NULL);
 }