Implement requirements about when readers should get locks assigned.
authordrepper <drepper>
Wed, 12 Jan 2000 11:33:25 +0000 (11:33 +0000)
committerdrepper <drepper>
Wed, 12 Jan 2000 11:33:25 +0000 (11:33 +0000)
linuxthreads/rwlock.c

index 1d78b78..6ea9aca 100644 (file)
 
 #include <errno.h>
 #include <pthread.h>
+#include <stdlib.h>
 #include "internals.h"
 #include "queue.h"
 #include "spinlock.h"
 #include "restart.h"
 
+/*
+ * Check whether the calling thread already owns one or more read locks on the
+ * specified lock. If so, return a pointer to the read lock info structure
+ * corresponding to that lock.
+ */
+
+static pthread_readlock_info *
+rwlock_is_in_list(pthread_descr self, pthread_rwlock_t *rwlock)
+{
+  pthread_readlock_info *info;
+
+  for (info = self->p_readlock_list; info != NULL; info = info->pr_next)
+    {
+      if (info->pr_lock == rwlock)
+       return info;
+    }
+
+  return NULL;
+}
+
+/*
+ * Add a new lock to the thread's list of locks for which it has a read lock.
+ * A new info node must be allocated for this, which is taken from the thread's
+ * free list, or by calling malloc. If malloc fails, a null pointer is
+ * returned. Otherwise the lock info structure is initialized and pushed
+ * onto the thread's list.
+ */
+
+static pthread_readlock_info *
+rwlock_add_to_list(pthread_descr self, pthread_rwlock_t *rwlock)
+{
+  pthread_readlock_info *info = self->p_readlock_free;
+
+  if (info != NULL) 
+    self->p_readlock_free = info->pr_next;
+  else
+    info = malloc(sizeof *info);
+
+  if (info == NULL)
+    return NULL;
+
+  info->pr_lock_count = 1;
+  info->pr_lock = rwlock;
+  info->pr_next = self->p_readlock_list;
+  self->p_readlock_list = info;
+
+  return info;
+}
+
+/*
+ * If the thread owns a read lock over the given pthread_rwlock_t,
+ * and this read lock is tracked in the thread's lock list,
+ * this function returns a pointer to the info node in that list.
+ * It also decrements the lock count within that node, and if
+ * it reaches zero, it removes the node from the list.
+ * If nothing is found, it returns a null pointer.
+ */
+
+static pthread_readlock_info *
+rwlock_remove_from_list(pthread_descr self, pthread_rwlock_t *rwlock)
+{
+  pthread_readlock_info **pinfo;
+
+  for (pinfo = &self->p_readlock_list; *pinfo != NULL; pinfo = &(*pinfo)->pr_next)
+    {
+      if ((*pinfo)->pr_lock == rwlock)
+       {
+         pthread_readlock_info *info = *pinfo;
+         if (--info->pr_lock_count == 0)
+           *pinfo = info->pr_next;
+         return info;
+       }
+    }
+  
+  return NULL;
+}
+
+/*
+ * This function checks whether the conditions are right to place a read lock.
+ * It returns 1 if so, otherwise zero. The rwlock's internal lock must be
+ * locked upon entry.
+ */
+
+static int
+rwlock_can_rdlock(pthread_rwlock_t *rwlock, int have_lock_already)
+{
+  /* Can't readlock; it is write locked. */
+  if (rwlock->__rw_writer != NULL)
+    return 0;
+
+  /* Lock prefers readers; get it. */
+  if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_READER_NP)
+    return 1;
+
+  /* Lock prefers writers, but none are waiting. */
+  if (queue_is_empty(&rwlock->__rw_write_waiting))
+    return 1;
+
+  /* Writers are waiting, but this thread already has a read lock */
+  if (have_lock_already)
+    return 1;
+
+  /* Writers are waiting, and this is a new lock */
+  return 0;
+}
+
+/*
+ * This function helps support brain-damaged recursive read locking
+ * semantics required by Unix 98, while maintaining write priority.
+ * This basically determines whether this thread already holds a read lock
+ * already. It returns 1 if so, otherwise it returns 0.  
+ *
+ * If the thread has any ``untracked read locks'' then it just assumes
+ * that this lock is among them, just to be safe, and returns 1.
+ *
+ * Also, if it finds the thread's lock in the list, it sets the pointer
+ * referenced by pexisting to refer to the list entry.
+ *
+ * If the thread has no untracked locks, and the lock is not found
+ * in its list, then it is added to the list. If this fails,
+ * then *pout_of_mem is set to 1.
+ */
+
+static int
+rwlock_have_already(pthread_descr *pself, pthread_rwlock_t *rwlock,
+    pthread_readlock_info **pexisting, int *pout_of_mem)
+{
+  pthread_readlock_info *existing = NULL;
+  int out_of_mem = 0, have_lock_already = 0;
+  pthread_descr self = *pself;
+
+  if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_WRITER_NP)
+    {
+      if (!self)
+       self = thread_self();
+
+      existing = rwlock_is_in_list(self, rwlock);
+
+      if (existing != NULL || self->p_untracked_readlock_count > 0)
+       have_lock_already = 1;
+      else
+       {
+         existing = rwlock_add_to_list(self, rwlock);
+         if (existing == NULL)
+           out_of_mem = 1;
+       }
+    }
+
+  *pout_of_mem = out_of_mem;
+  *pexisting = existing;
+  *pself = self;
+
+  return have_lock_already;
+}
+
 int
 pthread_rwlock_init (pthread_rwlock_t *rwlock,
                     const pthread_rwlockattr_t *attr)
@@ -68,24 +224,26 @@ pthread_rwlock_destroy (pthread_rwlock_t *rwlock)
   return 0;
 }
 
-
 int
 pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
 {
   pthread_descr self = NULL;
+  pthread_readlock_info *existing;
+  int out_of_mem, have_lock_already;
+
+  have_lock_already = rwlock_have_already(&self, rwlock,
+      &existing, &out_of_mem);
 
-  while (1)
+  for (;;)
     {
+      if (self == NULL)
+       self = thread_self ();
+
       __pthread_lock (&rwlock->__rw_lock, self);
-      if (rwlock->__rw_writer == NULL
-         || (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_READER_NP
-             && rwlock->__rw_readers != 0))
-       /* We can add a reader lock.  */
+
+      if (rwlock_can_rdlock(rwlock, have_lock_already))
        break;
 
-      /* Suspend ourselves, then try again */
-      if (self == NULL)
-       self = thread_self ();
       enqueue (&rwlock->__rw_read_waiting, self);
       __pthread_unlock (&rwlock->__rw_lock);
       suspend (self); /* This is not a cancellation point */
@@ -94,26 +252,50 @@ pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
   ++rwlock->__rw_readers;
   __pthread_unlock (&rwlock->__rw_lock);
 
+  if (have_lock_already || out_of_mem)
+    {
+      if (existing != NULL)
+       existing->pr_lock_count++;
+      else
+       self->p_untracked_readlock_count++;
+    }
+  
   return 0;
 }
 
-
 int
 pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
 {
-  int result = EBUSY;
+  pthread_descr self = thread_self();
+  pthread_readlock_info *existing;
+  int out_of_mem, have_lock_already;
+  int retval = EBUSY;
 
-  __pthread_lock (&rwlock->__rw_lock, NULL);
-  if (rwlock->__rw_writer == NULL
-      || (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_READER_NP
-         && rwlock->__rw_readers != 0))
+  have_lock_already = rwlock_have_already(&self, rwlock,
+      &existing, &out_of_mem);
+
+  __pthread_lock (&rwlock->__rw_lock, self);
+
+  if (rwlock_can_rdlock(rwlock, have_lock_already))
     {
       ++rwlock->__rw_readers;
-      result = 0;
+      retval = 0;
     }
+
   __pthread_unlock (&rwlock->__rw_lock);
 
-  return result;
+  if (retval == 0)
+    {
+      if (have_lock_already || out_of_mem)
+       {
+         if (existing != NULL)
+           existing->pr_lock_count++;
+         else
+           self->p_untracked_readlock_count++;
+       }
+    }
+  
+  return retval;
 }
 
 
@@ -210,6 +392,28 @@ pthread_rwlock_unlock (pthread_rwlock_t *rwlock)
       __pthread_unlock (&rwlock->__rw_lock);
       if (th != NULL)
        restart (th);
+
+      /* Recursive lock fixup */
+
+      if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_WRITER_NP)
+       {
+         pthread_descr self = thread_self();
+         pthread_readlock_info *victim = rwlock_remove_from_list(self, rwlock);
+
+         if (victim != NULL)
+           {
+             if (victim->pr_lock_count == 0)
+               {
+                 victim->pr_next = self->p_readlock_free;
+                 self->p_readlock_free = victim;
+               }
+           }
+         else
+           {
+             if (self->p_untracked_readlock_count > 0)
+               self->p_untracked_readlock_count--;
+           }
+       }
     }
 
   return 0;