[PATCH 2/2] locking/qrwlock: Reduce cacheline contention for rwlocks used in interrupt context

[Waiman Long]

Even though qrwlock is supposed to be a fair lock, it does allow readers
from interrupt context to spin on the lock until it can acquire it making
it not as fair. This exception was added due to the requirement to allow
recursive read lock in interrupt context. This can also be achieved by
just ignoring the writer waiting bit without spinning on the lock.

By making this change, we make qrwlock a bit more fair and eliminating
the problem of cacheline bouncing for rwlocks that are used heavily in
interrupt context, like the networking stack. This should also reduce
the chance of lock starvation for those interrupt context rwlocks.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 include/asm-generic/qrwlock.h |  6 +++---
 kernel/locking/qrwlock.c      | 17 ++++++-----------
 2 files changed, 9 insertions(+), 14 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index d4cd4cc4389c..9d40cf016e0c 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -29,7 +29,7 @@
 /*
  * External function declarations
  */
-extern void queued_read_lock_slowpath(struct qrwlock *lock);
+extern void queued_read_lock_slowpath(struct qrwlock *lock, int cnts);
 extern void queued_write_lock_slowpath(struct qrwlock *lock);
 
 /**
@@ -80,7 +80,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
 		return;
 
 	/* The slowpath will decrement the reader count, if necessary. */
-	queued_read_lock_slowpath(lock);
+	queued_read_lock_slowpath(lock, cnts);
 }
 
 /**
@@ -90,7 +90,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
 static inline void queued_write_lock(struct qrwlock *lock)
 {
 	int cnts = 0;
-	/* Optimize for the unfair lock case where the fair flag is 0. */
+
 	if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
 		return;
 
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 2e1600906c9f..d52d13e95600 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -18,21 +18,16 @@
  * queued_read_lock_slowpath - acquire read lock of a queued rwlock
  * @lock: Pointer to queued rwlock structure
  */
-void queued_read_lock_slowpath(struct qrwlock *lock)
+void queued_read_lock_slowpath(struct qrwlock *lock, int cnts)
 {
 	/*
-	 * Readers come here when they cannot get the lock without waiting
+	 * Readers come here when they cannot get the lock without waiting.
+	 * Readers in interrupt context can steal the lock immediately
+	 * if the writer is just waiting (not holding the lock yet).
 	 */
-	if (unlikely(in_interrupt())) {
-		/*
-		 * Readers in interrupt context will get the lock immediately
-		 * if the writer is just waiting (not holding the lock yet),
-		 * so spin with ACQUIRE semantics until the lock is available
-		 * without waiting in the queue.
-		 */
-		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
+	if (unlikely(!(cnts & _QW_LOCKED) && in_interrupt()))
 		return;
-	}
+
 	atomic_sub(_QR_BIAS, &lock->cnts);
 
 	trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);
-- 
2.27.0