coroutine-lock: Reimplement CoRwlock to fix downgrade bug

An invariant of the current rwlock is that if multiple coroutines hold a
reader lock, all must be runnable. The unlock implementation relies on
this, choosing to wake a single coroutine when the final read lock
holder exits the critical section, assuming that it will wake a
coroutine attempting to acquire a write lock.

The downgrade implementation violates this assumption by creating a
read lock owning coroutine that is exclusively runnable - any other
coroutines that are waiting to acquire a read lock are *not* made
runnable when the write lock holder converts its ownership to read
only.

More in general, the old implementation had lots of other fairness bugs.
The root cause of the bugs was that CoQueue would wake up readers even
if there were pending writers, and would wake up writers even if there
were readers.  In that case, the coroutine would go back to sleep *at
the end* of the CoQueue, losing its place at the head of the line.

To fix this, keep the queue of waiters explicitly in the CoRwlock
instead of using CoQueue, and store for each whether it is a
potential reader or a writer.  This way, downgrade can look at the
first queued coroutines and wake it only if it is a reader, causing
all other readers in line to be released in turn.

Reported-by: David Edmondson <david.edmondson@oracle.com>
Reviewed-by: David Edmondson <david.edmondson@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210325112941.365238-5-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 10 insertions, 7 deletions

diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index 84eab6e3bf..ce5b9c6851 100644
--- a/include/qemu/coroutine.h
+++ b/include/qemu/coroutine.h
@@ -237,11 +237,15 @@ bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock);
 bool qemu_co_queue_empty(CoQueue *queue);
 
 
+typedef struct CoRwTicket CoRwTicket;
 typedef struct CoRwlock {
-    int pending_writer;
-    int reader;
     CoMutex mutex;
-    CoQueue queue;
+
+    /* Number of readers, or -1 if owned for writing.  */
+    int owners;
+
+    /* Waiting coroutines.  */
+    QSIMPLEQ_HEAD(, CoRwTicket) tickets;
 } CoRwlock;
 
 /**
@@ -260,10 +264,9 @@ void qemu_co_rwlock_rdlock(CoRwlock *lock);
 /**
  * Write Locks the CoRwlock from a reader.  This is a bit more efficient than
  * @qemu_co_rwlock_unlock followed by a separate @qemu_co_rwlock_wrlock.
- * However, if the lock cannot be upgraded immediately, control is transferred
- * to the caller of the current coroutine.  Also, @qemu_co_rwlock_upgrade
- * only overrides CoRwlock fairness if there are no concurrent readers, so
- * another writer might run while @qemu_co_rwlock_upgrade blocks.
+ * Note that if the lock cannot be upgraded immediately, control is transferred
+ * to the caller of the current coroutine; another writer might run while
+ * @qemu_co_rwlock_upgrade blocks.
  */
 void qemu_co_rwlock_upgrade(CoRwlock *lock);