+================
Futex Requeue PI
-----------------
+================
Requeueing of tasks from a non-PI futex to a PI futex requires
special handling in order to ensure the underlying rt_mutex is never
rest to the natural wakeup inherent in unlocking the mutex
associated with the condvar.
-Consider the simplified glibc calls:
-
-/* caller must lock mutex */
-pthread_cond_wait(cond, mutex)
-{
- lock(cond->__data.__lock);
- unlock(mutex);
- do {
- unlock(cond->__data.__lock);
- futex_wait(cond->__data.__futex);
- lock(cond->__data.__lock);
- } while(...)
- unlock(cond->__data.__lock);
- lock(mutex);
-}
-
-pthread_cond_broadcast(cond)
-{
- lock(cond->__data.__lock);
- unlock(cond->__data.__lock);
- futex_requeue(cond->data.__futex, cond->mutex);
-}
+Consider the simplified glibc calls::
+
+ /* caller must lock mutex */
+ pthread_cond_wait(cond, mutex)
+ {
+ lock(cond->__data.__lock);
+ unlock(mutex);
+ do {
+ unlock(cond->__data.__lock);
+ futex_wait(cond->__data.__futex);
+ lock(cond->__data.__lock);
+ } while(...)
+ unlock(cond->__data.__lock);
+ lock(mutex);
+ }
+
+ pthread_cond_broadcast(cond)
+ {
+ lock(cond->__data.__lock);
+ unlock(cond->__data.__lock);
+ futex_requeue(cond->data.__futex, cond->mutex);
+ }
Once pthread_cond_broadcast() requeues the tasks, the cond->mutex
has waiters. Note that pthread_cond_wait() attempts to lock the
be able to requeue tasks to PI futexes. This support implies that
upon a successful futex_wait system call, the caller would return to
user space already holding the PI futex. The glibc implementation
-would be modified as follows:
-
-
-/* caller must lock mutex */
-pthread_cond_wait_pi(cond, mutex)
-{
- lock(cond->__data.__lock);
- unlock(mutex);
- do {
- unlock(cond->__data.__lock);
- futex_wait_requeue_pi(cond->__data.__futex);
- lock(cond->__data.__lock);
- } while(...)
- unlock(cond->__data.__lock);
- /* the kernel acquired the mutex for us */
-}
-
-pthread_cond_broadcast_pi(cond)
-{
- lock(cond->__data.__lock);
- unlock(cond->__data.__lock);
- futex_requeue_pi(cond->data.__futex, cond->mutex);
-}
+would be modified as follows::
+
+
+ /* caller must lock mutex */
+ pthread_cond_wait_pi(cond, mutex)
+ {
+ lock(cond->__data.__lock);
+ unlock(mutex);
+ do {
+ unlock(cond->__data.__lock);
+ futex_wait_requeue_pi(cond->__data.__futex);
+ lock(cond->__data.__lock);
+ } while(...)
+ unlock(cond->__data.__lock);
+ /* the kernel acquired the mutex for us */
+ }
+
+ pthread_cond_broadcast_pi(cond)
+ {
+ lock(cond->__data.__lock);
+ unlock(cond->__data.__lock);
+ futex_requeue_pi(cond->data.__futex, cond->mutex);
+ }
The actual glibc implementation will likely test for PI and make the
necessary changes inside the existing calls rather than creating new