lib/uklock: Remove the rwlock_{upgrade,downgrade} functions

In the current form they impose very strong synchronization requirements,
making it impossible to implement a more efficient read-write lock design.

Most rwlocks sidestep this by either using a weaker `try_upgrade` variant
or having a separate `lock_upgradable` function (allowing multiple readers
but only one upgradable lock/writer lock).

This removes these two functions for now to prevent new code relying on
these functions. This change is breaking because it already was part of the
previous release.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #1090

lib/uklock/include/uk/rwlock.h

@@ -90,24 +90,6 @@ void uk_rwlock_runlock(struct uk_rwlock *rwl);
  */
 void uk_rwlock_wunlock(struct uk_rwlock *rwl);
 
-/**
- * Upgrade the given read lock to a write lock
- *
- * @param rwl
- *   Reader-writer lock to upgrade. The current thread must hold the lock
- *   for reading
- */
-void uk_rwlock_upgrade(struct uk_rwlock *rwl);
-
-/**
- * Downgrade the given write lock to a read lock
- *
- * @param rwl
- *   Reader-writer lock to downgrade. The current thread must hold the lock
- *   for writing
- */
-void uk_rwlock_downgrade(struct uk_rwlock *rwl);
-
 
 #ifdef __cplusplus
 }

lib/uklock/rwlock.c

@@ -133,60 +133,3 @@ void uk_rwlock_wunlock(struct uk_rwlock *rwl)
 		uk_waitq_wake_up_one(&rwl->exclusive);
 }
 
-void uk_rwlock_upgrade(struct uk_rwlock *rwl)
-{
-	UK_ASSERT(rwl);
-
-	uk_spin_lock(&rwl->sl);
-	if (rwl->nactive == 1) {
-		/* We are the only active reader. Just upgrade to writer */
-		rwl->nactive = -1;
-	} else {
-		/* There are other readers. Wait until these have left */
-		UK_ASSERT(rwl->nactive > 1);
-
-		/*
-		 * Indicate that we are waiting for write access and remove
-		 * this thread from the active readers.
-		 */
-		rwl->npending_writes++;
-		rwl->nactive--;
-
-		uk_waitq_wait_event_locked(&rwl->shared,
-					   rwl->nactive == 0,
-					   uk_spin_lock,
-					   uk_spin_unlock,
-					   &rwl->sl);
-
-		UK_ASSERT(rwl->npending_writes > 0);
-		UK_ASSERT(rwl->nactive == 0);
-
-		/* We are now the writer. Remove the satisfied request and mark
-		 * the lock for write access.
-		 */
-		rwl->npending_writes--;
-		rwl->nactive = -1;
-	}
-	uk_spin_unlock(&rwl->sl);
-}
-
-void uk_rwlock_downgrade(struct uk_rwlock *rwl)
-{
-	int wake_readers;
-
-	UK_ASSERT(rwl);
-
-	uk_spin_lock(&rwl->sl);
-	UK_ASSERT(rwl->nactive == -1);
-
-	/* We are the writer. Downgrade the lock to read access by
-	 * transforming to a reader. If there are other readers waiting, wake
-	 * them up.
-	 */
-	rwl->nactive = 1;
-	wake_readers = (rwl->npending_reads > 0);
-	uk_spin_unlock(&rwl->sl);
-
-	if (wake_readers)
-		uk_waitq_wake_up(&rwl->shared);
-}