pthread_cond_wait: Add mutex to protect the waiter count

The load/compare and RMW to wait_count need protection. Bumping the
counter can be done by atomic_fetch_add as there is no race here.
However reading the counter when signaling needs to take a lock so that
if multiple threads signal at the same time, only one has exclusive
access to the counter.

NOTE:
The assumption that the user will call pthread_cond_signal /
pthread_cond_broadcast with the mutex given to pthread_cond_wait held is
simply not true. It MAY hold it, but it is not forced. Thus, using the
user space lock for protecting the wait counter as well is not valid!

The pthread_cond_signal() or pthread_cond_broadcast() functions may be called by a thread whether or not it currently owns the mutex that threads calling pthread_cond_wait() or pthread_cond_timedwait() have associated with the condition variable during their waits; however, if predictable scheduling behaviour is required, then that mutex is locked by the thread calling pthread_cond_signal() or pthread_cond_broadcast().

[1] https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_signal.html

include/pthread.h

@@ -269,7 +269,8 @@ struct pthread_cond_s
 {
   sem_t sem;
   clockid_t clockid;
-  uint16_t wait_count;
+  unsigned int wait_count;
+  mutex_t mutex;
 };
 
 #ifndef __PTHREAD_COND_T_DEFINED

libs/libc/pthread/pthread_condinit.c

@@ -31,6 +31,8 @@
 #include <debug.h>
 #include <errno.h>
 
+#include <nuttx/atomic.h>
+
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
@@ -75,6 +77,7 @@ int pthread_cond_init(FAR pthread_cond_t *cond,
     {
       cond->clockid = attr ? attr->clockid : CLOCK_REALTIME;
       cond->wait_count = 0;
+      nxmutex_init(&cond->mutex);
     }
 
   sinfo("Returning %d\n", ret);

sched/pthread/pthread_condbroadcast.c

@@ -31,6 +31,8 @@
 #include <errno.h>
 #include <debug.h>
 
+#include <nuttx/atomic.h>
+
 #include "pthread/pthread.h"
 
 /****************************************************************************
@@ -42,10 +44,7 @@
  *
  * Description:
  *    A thread broadcast on a condition variable.
- *    pthread_cond_broadcast shall unblock all threads currently blocked on a
- *    specified condition variable cond. We need own the mutex that threads
- *    calling pthread_cond_wait or pthread_cond_timedwait have associated
- *    with the condition variable during their wait.
+ *
  * Input Parameters:
  *   None
  *
@@ -68,9 +67,11 @@ int pthread_cond_broadcast(FAR pthread_cond_t *cond)
     }
   else
     {
+      nxmutex_lock(&cond->mutex);
+
       /* Loop until all of the waiting threads have been restarted. */
 
-      while (cond->wait_count > 0)
+      while (atomic_load(&cond->wait_count) > 0)
         {
           /* If the value is less than zero (meaning that one or more
            * thread is waiting), then post the condition semaphore.
@@ -83,8 +84,10 @@ int pthread_cond_broadcast(FAR pthread_cond_t *cond)
            * above post).
            */
 
-          cond->wait_count--;
+          atomic_fetch_sub(&cond->wait_count, 1);
         }
+
+      nxmutex_unlock(&cond->mutex);
     }
 
   sinfo("Returning %d\n", ret);

sched/pthread/pthread_condclockwait.c

@@ -36,6 +36,7 @@
 #include <assert.h>
 #include <debug.h>
 
+#include <nuttx/atomic.h>
 #include <nuttx/irq.h>
 #include <nuttx/wdog.h>
 #include <nuttx/signal.h>
@@ -113,7 +114,7 @@ int pthread_cond_clockwait(FAR pthread_cond_t *cond,
 
       sinfo("Give up mutex...\n");
 
-      cond->wait_count++;
+      atomic_fetch_add(&cond->wait_count, 1);
 
       /* Give up the mutex */
 

sched/pthread/pthread_condsignal.c

@@ -30,6 +30,8 @@
 #include <errno.h>
 #include <debug.h>
 
+#include <nuttx/atomic.h>
+
 #include "pthread/pthread.h"
 
 /****************************************************************************
@@ -41,10 +43,6 @@
  *
  * Description:
  *    A thread can signal on a condition variable.
- *    pthread_cond_signal shall unblock a thread currently blocked on a
- *    specified condition variable cond. We need own the mutex that threads
- *    calling pthread_cond_wait or pthread_cond_timedwait have associated
- *    with the condition variable during their wait.
  *
  * Input Parameters:
  *   None
@@ -68,12 +66,16 @@ int pthread_cond_signal(FAR pthread_cond_t *cond)
     }
   else
     {
-      if (cond->wait_count > 0)
+      nxmutex_lock(&cond->mutex);
+
+      if (atomic_load(&cond->wait_count) > 0)
         {
           sinfo("Signalling...\n");
-          cond->wait_count--;
+          atomic_fetch_sub(&cond->wait_count, 1);
           ret = -nxsem_post(&cond->sem);
         }
+
+      nxmutex_unlock(&cond->mutex);
     }
 
   sinfo("Returning %d\n", ret);

sched/pthread/pthread_condwait.c

@@ -32,6 +32,7 @@
 #include <errno.h>
 #include <debug.h>
 
+#include <nuttx/atomic.h>
 #include <nuttx/cancelpt.h>
 
 #include "pthread/pthread.h"
@@ -88,7 +89,7 @@ int pthread_cond_wait(FAR pthread_cond_t *cond, FAR pthread_mutex_t *mutex)
 
       sinfo("Give up mutex / take cond\n");
 
-      cond->wait_count++;
+      atomic_fetch_add(&cond->wait_count, 1);
       ret = pthread_mutex_breaklock(mutex, &nlocks);
 
       status = -nxsem_wait_uninterruptible(&cond->sem);