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authorSebastian Huber <sebastian.huber@embedded-brains.de>2015-09-28 07:29:11 +0200
committerSebastian Huber <sebastian.huber@embedded-brains.de>2015-09-28 07:45:54 +0200
commitaee6a1d05fc660b7177ded38d924757357925f1c (patch)
tree2ff24043fca4f15e9d424ee2c9d64f2d88b54654
parent8d296cd54e51d2fe1f6bf5344cca70c822b04568 (diff)
downloadrtems-aee6a1d05fc660b7177ded38d924757357925f1c.tar.bz2
SMP: Simplify thread lock operations
-rw-r--r--cpukit/score/include/rtems/score/threadimpl.h52
1 files changed, 25 insertions, 27 deletions
diff --git a/cpukit/score/include/rtems/score/threadimpl.h b/cpukit/score/include/rtems/score/threadimpl.h
index 1092b65f9b..68c26c388f 100644
--- a/cpukit/score/include/rtems/score/threadimpl.h
+++ b/cpukit/score/include/rtems/score/threadimpl.h
@@ -1168,16 +1168,19 @@ RTEMS_INLINE_ROUTINE void *_Thread_Lock_acquire(
SMP_ticket_lock_Control *lock;
while ( true ) {
- uint32_t my_generation;
+ unsigned int first_generation;
+ unsigned int second_generation;
_ISR_lock_ISR_disable( lock_context );
- my_generation = the_thread->Lock.generation;
/*
- * Ensure that we read the initial lock generation before we obtain our
- * current lock.
+ * Ensure that we read our first lock generation before we obtain our
+ * current lock. See _Thread_Lock_set_unprotected().
*/
- _Atomic_Fence( ATOMIC_ORDER_ACQUIRE );
+ first_generation = _Atomic_Load_uint(
+ &the_thread->Lock.generation,
+ ATOMIC_ORDER_ACQUIRE
+ );
lock = the_thread->Lock.current;
_SMP_ticket_lock_Acquire(
@@ -1187,19 +1190,22 @@ RTEMS_INLINE_ROUTINE void *_Thread_Lock_acquire(
);
/*
- * Ensure that we read the second lock generation after we obtained our
- * current lock.
+ * The C11 memory model doesn't guarantee that we read the latest
+ * generation here. For this a read-modify-write operation would be
+ * necessary. We read at least the new generation set up by the owner of
+ * our current thread lock, and so on.
*/
- _Atomic_Fence( ATOMIC_ORDER_ACQUIRE );
+ second_generation = _Atomic_Load_uint(
+ &the_thread->Lock.generation,
+ ATOMIC_ORDER_ACQUIRE
+ );
- if ( the_thread->Lock.generation == my_generation ) {
- break;
+ if ( first_generation == second_generation ) {
+ return lock;
}
_Thread_Lock_release( lock, lock_context );
}
-
- return lock;
#else
_ISR_Disable( lock_context->isr_level );
@@ -1220,20 +1226,19 @@ RTEMS_INLINE_ROUTINE void _Thread_Lock_set_unprotected(
the_thread->Lock.current = new_lock;
/*
- * Ensure that the new lock is visible before we update the generation
- * number. Otherwise someone would be able to read an up to date generation
- * number and an old lock.
- */
- _Atomic_Fence( ATOMIC_ORDER_RELEASE );
-
- /*
+ * The generation release corresponds to the generation acquire in
+ * _Thread_Lock_acquire() and ensures that the new lock and other fields are
+ * visible to the next thread lock owner. Otherwise someone would be able to
+ * read an up to date generation number and an old lock. See
+ * _Thread_Wait_set_queue() and _Thread_Wait_restore_default_operations().
+ *
* Since we set a new lock right before, this increment is not protected by a
* lock and thus must be an atomic operation.
*/
_Atomic_Fetch_add_uint(
&the_thread->Lock.generation,
1,
- ATOMIC_ORDER_RELAXED
+ ATOMIC_ORDER_RELEASE
);
}
#endif
@@ -1277,13 +1282,6 @@ RTEMS_INLINE_ROUTINE void _Thread_Lock_restore_default(
Thread_Control *the_thread
)
{
- /*
- * Ensures that the stores to the wait queue and operations completed before
- * the default lock is restored. See _Thread_Wait_set_queue() and
- * _Thread_Wait_restore_default_operations().
- */
- _Atomic_Fence( ATOMIC_ORDER_RELEASE );
-
_Thread_Lock_set_unprotected( the_thread, &the_thread->Lock.Default );
}
#else