[PATCH v2 5/5] kernel/locking: Prevent slowpath writers getting held up by fastpath

[Will Deacon]

When a prospective writer takes the qrwlock locking slowpath due to the
lock being held, it attempts to cmpxchg the wmode field from 0 to
_QW_WAITING so that concurrent lockers also take the slowpath and queue
on the spinlock accordingly, allowing the lockers to drain.

Unfortunately, this isn't fair, because a fastpath writer that comes in
after the lock is made available but before the _QW_WAITING flag is set
can effectively jump the queue. If there is a steady stream of prospective
writers, then the waiter will be held off indefinitely.

This patch restores fairness by separating _QW_WAITING and _QW_LOCKED
into two distinct fields: _QW_LOCKED continues to occupy the bottom byte
of the lockword so that it can be cleared unconditionally when unlocking,
but _QW_WAITING now occupies what used to be the bottom bit of the reader
count. This then forces the slow-path for concurrent lockers.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 include/asm-generic/qrwlock.h       | 10 +++++-----
 include/asm-generic/qrwlock_types.h |  8 ++++----
 kernel/locking/qrwlock.c            | 20 +++++---------------
 3 files changed, 14 insertions(+), 24 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 02c0a768e6b0..63cb7d347b25 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -40,10 +40,10 @@
  *       |      rd      | wr |
  *       +----+----+----+----+
  */
-#define	_QW_WAITING	1		/* A writer is waiting	   */
-#define	_QW_LOCKED	0xff		/* A writer holds the lock */
-#define	_QW_WMASK	0xff		/* Writer mask		   */
-#define	_QR_SHIFT	8		/* Reader count shift	   */
+#define	_QW_WAITING	0x100		/* A writer is waiting	   */
+#define	_QW_LOCKED	0x0ff		/* A writer holds the lock */
+#define	_QW_WMASK	0x1ff		/* Writer mask		   */
+#define	_QR_SHIFT	9		/* Reader count shift	   */
 #define _QR_BIAS	(1U << _QR_SHIFT)
 
 /*
@@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
  */
 static inline void queued_write_unlock(struct qrwlock *lock)
 {
-	smp_store_release(&lock->wmode, 0);
+	smp_store_release(&lock->wlocked, 0);
 }
 
 /*
diff --git a/include/asm-generic/qrwlock_types.h b/include/asm-generic/qrwlock_types.h
index 507f2dc51bba..8af752acbdc0 100644
--- a/include/asm-generic/qrwlock_types.h
+++ b/include/asm-generic/qrwlock_types.h
@@ -13,11 +13,11 @@ typedef struct qrwlock {
 		atomic_t cnts;
 		struct {
 #ifdef __LITTLE_ENDIAN
-			u8 wmode;	/* Writer mode   */
-			u8 rcnts[3];	/* Reader counts */
+			u8 wlocked;	/* Locked for write? */
+			u8 __lstate[3];
 #else
-			u8 rcnts[3];	/* Reader counts */
-			u8 wmode;	/* Writer mode   */
+			u8 __lstate[3];
+			u8 wlocked;	/* Locked for write? */
 #endif
 		};
 	};
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index b7ea4647c74d..e940f2c2b4f2 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -40,8 +40,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
 		 * so spin with ACQUIRE semantics until the lock is available
 		 * without waiting in the queue.
 		 */
-		atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK)
-					 != _QW_LOCKED);
+		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
 		return;
 	}
 	atomic_sub(_QR_BIAS, &lock->cnts);
@@ -57,7 +56,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
 	 * that accesses can't leak upwards out of our subsequent critical
 	 * section in the case that the lock is currently held for write.
 	 */
-	atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK) != _QW_LOCKED);
+	atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
 
 	/*
 	 * Signal the next one in queue to become queue head
@@ -80,19 +79,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
 	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
 		goto unlock;
 
-	/*
-	 * Set the waiting flag to notify readers that a writer is pending,
-	 * or wait for a previous writer to go away.
-	 */
-	for (;;) {
-		if (!READ_ONCE(lock->wmode) &&
-		   (cmpxchg_relaxed(&lock->wmode, 0, _QW_WAITING) == 0))
-			break;
-
-		cpu_relax();
-	}
+	/* Set the waiting flag to notify readers that a writer is pending */
+	atomic_add(_QW_WAITING, &lock->cnts);
 
-	/* When no more readers, set the locked flag */
+	/* When no more readers or writers, set the locked flag */
 	do {
 		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
 	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
-- 
2.1.4