locking/rwsem: Implement a new locking scheme

The current way of using various reader, writer and waiting biases
in the rwsem code are confusing and hard to understand. I have to
reread the rwsem count guide in the rwsem-xadd.c file from time to
time to remind myself how this whole thing works. It also makes the
rwsem code harder to be optimized.

To make rwsem more sane, a new locking scheme similar to the one in
qrwlock is now being used.  The atomic long count has the following
bit definitions:

  Bit  0   - writer locked bit
  Bit  1   - waiters present bit
  Bits 2-7 - reserved for future extension
  Bits 8-X - reader count (24/56 bits)

The cmpxchg instruction is now used to acquire the write lock. The read
lock is still acquired with xadd instruction, so there is no change here.
This scheme will allow up to 16M/64P active readers which should be
more than enough. We can always use some more reserved bits if necessary.

With that change, we can deterministically know if a rwsem has been
write-locked. Looking at the count alone, however, one cannot determine
for certain if a rwsem is owned by readers or not as the readers that
set the reader count bits may be in the process of backing out. So we
still need the reader-owned bit in the owner field to be sure.

With a locking microbenchmark running on 5.1 based kernel, the total
locking rates (in kops/s) of the benchmark on a 8-socket 120-core
IvyBridge-EX system before and after the patch were as follows:

                  Before Patch      After Patch
   # of Threads  wlock    rlock    wlock    rlock
   ------------  -----    -----    -----    -----
        1        30,659   31,341   31,055   31,283
        2         8,909   16,457    9,884   17,659
        4         9,028   15,823    8,933   20,233
        8         8,410   14,212    7,230   17,140
       16         8,217   25,240    7,479   24,607

The locking rates of the benchmark on a Power8 system were as follows:

                  Before Patch      After Patch
   # of Threads  wlock    rlock    wlock    rlock
   ------------  -----    -----    -----    -----
        1        12,963   13,647   13,275   13,601
        2         7,570   11,569    7,902   10,829
        4         5,232    5,516    5,466    5,435
        8         5,233    3,386    5,467    3,168

The locking rates of the benchmark on a 2-socket ARM64 system were
as follows:

                  Before Patch      After Patch
   # of Threads  wlock    rlock    wlock    rlock
   ------------  -----    -----    -----    -----
        1        21,495   21,046   21,524   21,074
        2         5,293   10,502    5,333   10,504
        4         5,325   11,463    5,358   11,631
        8         5,391   11,712    5,470   11,680

The performance are roughly the same before and after the patch. There
are run-to-run variations in performance. Runs with higher variances
usually have higher throughput.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: huang ying <huang.ying.caritas@gmail.com>
Link: https://lkml.kernel.org/r/20190520205918.22251-4-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 36 insertions, 38 deletions

diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index eb9c8534299b..499a9b2bda82 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -42,24 +42,24 @@
 #endif
 
 /*
- * R/W semaphores originally for PPC using the stuff in lib/rwsem.c.
- * Adapted largely from include/asm-i386/rwsem.h
- * by Paul Mackerras <paulus@samba.org>.
- */
-
-/*
- * the semaphore definition
+ * The definition of the atomic counter in the semaphore:
+ *
+ * Bit  0   - writer locked bit
+ * Bit  1   - waiters present bit
+ * Bits 2-7 - reserved
+ * Bits 8-X - 24-bit (32-bit) or 56-bit reader count
+ *
+ * atomic_long_fetch_add() is used to obtain reader lock, whereas
+ * atomic_long_cmpxchg() will be used to obtain writer lock.
  */
-#ifdef CONFIG_64BIT
-# define RWSEM_ACTIVE_MASK		0xffffffffL
-#else
-# define RWSEM_ACTIVE_MASK		0x0000ffffL
-#endif
-
-#define RWSEM_ACTIVE_BIAS		0x00000001L
-#define RWSEM_WAITING_BIAS		(-RWSEM_ACTIVE_MASK-1)
-#define RWSEM_ACTIVE_READ_BIAS		RWSEM_ACTIVE_BIAS
-#define RWSEM_ACTIVE_WRITE_BIAS		(RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+#define RWSEM_WRITER_LOCKED	(1UL << 0)
+#define RWSEM_FLAG_WAITERS	(1UL << 1)
+#define RWSEM_READER_SHIFT	8
+#define RWSEM_READER_BIAS	(1UL << RWSEM_READER_SHIFT)
+#define RWSEM_READER_MASK	(~(RWSEM_READER_BIAS - 1))
+#define RWSEM_WRITER_MASK	RWSEM_WRITER_LOCKED
+#define RWSEM_LOCK_MASK		(RWSEM_WRITER_MASK|RWSEM_READER_MASK)
+#define RWSEM_READ_FAILED_MASK	(RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS)
 
 /*
  * All writes to owner are protected by WRITE_ONCE() to make sure that
@@ -151,7 +151,8 @@ extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
  */
 static inline void __down_read(struct rw_semaphore *sem)
 {
-	if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0)) {
+	if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
+			&sem->count) & RWSEM_READ_FAILED_MASK)) {
 		rwsem_down_read_failed(sem);
 		DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
 					RWSEM_READER_OWNED), sem);
@@ -162,7 +163,8 @@ static inline void __down_read(struct rw_semaphore *sem)
 
 static inline int __down_read_killable(struct rw_semaphore *sem)
 {
-	if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0)) {
+	if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
+			&sem->count) & RWSEM_READ_FAILED_MASK)) {
 		if (IS_ERR(rwsem_down_read_failed_killable(sem)))
 			return -EINTR;
 		DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
@@ -183,11 +185,11 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 	lockevent_inc(rwsem_rtrylock);
 	do {
 		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
-					tmp + RWSEM_ACTIVE_READ_BIAS)) {
+					tmp + RWSEM_READER_BIAS)) {
 			rwsem_set_reader_owned(sem);
 			return 1;
 		}
-	} while (tmp >= 0);
+	} while (!(tmp & RWSEM_READ_FAILED_MASK));
 	return 0;
 }
 
@@ -196,22 +198,16 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
  */
 static inline void __down_write(struct rw_semaphore *sem)
 {
-	long tmp;
-
-	tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
-					     &sem->count);
-	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+	if (unlikely(atomic_long_cmpxchg_acquire(&sem->count, 0,
+						 RWSEM_WRITER_LOCKED)))
 		rwsem_down_write_failed(sem);
 	rwsem_set_owner(sem);
 }
 
 static inline int __down_write_killable(struct rw_semaphore *sem)
 {
-	long tmp;
-
-	tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
-					     &sem->count);
-	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+	if (unlikely(atomic_long_cmpxchg_acquire(&sem->count, 0,
+						 RWSEM_WRITER_LOCKED)))
 		if (IS_ERR(rwsem_down_write_failed_killable(sem)))
 			return -EINTR;
 	rwsem_set_owner(sem);
@@ -224,7 +220,7 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
 
 	lockevent_inc(rwsem_wtrylock);
 	tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
-		      RWSEM_ACTIVE_WRITE_BIAS);
+					  RWSEM_WRITER_LOCKED);
 	if (tmp == RWSEM_UNLOCKED_VALUE) {
 		rwsem_set_owner(sem);
 		return true;
@@ -242,8 +238,9 @@ static inline void __up_read(struct rw_semaphore *sem)
 	DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED),
 				sem);
 	rwsem_clear_reader_owned(sem);
-	tmp = atomic_long_dec_return_release(&sem->count);
-	if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
+	tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
+	if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS))
+			== RWSEM_FLAG_WAITERS))
 		rwsem_wake(sem);
 }
 
@@ -254,8 +251,8 @@ static inline void __up_write(struct rw_semaphore *sem)
 {
 	DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
 	rwsem_clear_owner(sem);
-	if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
-						    &sem->count) < 0))
+	if (unlikely(atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED,
+			&sem->count) & RWSEM_FLAG_WAITERS))
 		rwsem_wake(sem);
 }
 
@@ -274,8 +271,9 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
 	 * write side. As such, rely on RELEASE semantics.
 	 */
 	DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
-	tmp = atomic_long_add_return_release(-RWSEM_WAITING_BIAS, &sem->count);
+	tmp = atomic_long_fetch_add_release(
+		-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
 	rwsem_set_reader_owned(sem);
-	if (tmp < 0)
+	if (tmp & RWSEM_FLAG_WAITERS)
 		rwsem_downgrade_wake(sem);
 }