Date:   Thu,  1 Jun 2023 12:13:11 +0900
From:   Bongkyu Kim <bongkyu7.kim@...sung.com>
To:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, jwook1.kim@...sung.com,
        lakkyung.jung@...sung.com, Bongkyu Kim <bongkyu7.kim@...sung.com>,
        kernel test robot <lkp@...el.com>
Subject: [PATCH v2] locking/rwsem: Optionally re-enable reader optimistic
 spinning

Remove reader optimistic spinning has a regression on application
startup performance in android device. In mobile environment, reader
optimistic spinning is still useful because there're not many readers.
So re-enable reader optimistic spinning and disabled by default. And,
can turn on by cmdline.

This reverts commit 617f3ef95177 ("locking/rwsem: Remove reader optimistic
spinning"). And, the below changes are added.
- Add rwsem.opt_rspin cmdline
- Fix compile error without CONFIG_RWSEM_SPIN_ON_OWNER (reported by
  kernel test robot)

Test result:
This is 15 application startup performance in our s5e8535 soc.
- Cortex A78*2 + Cortex A55*6
- unit: ms (lower is better)

Application             base  opt_rspin  Diff  Diff(%)
--------------------  ------  ---------  ----  -------
* Total(geomean)         343        330   -13    +3.8%
--------------------  ------  ---------  ----  -------
helloworld               110        108    -2    +1.8%
Amazon_Seller            397        388    -9    +2.3%
Whatsapp                 311        304    -7    +2.3%
Simple_PDF_Reader        500        463   -37    +7.4%
FaceApp                  330        317   -13    +3.9%
Timestamp_Camera_Free    451        443    -8    +1.8%
Kindle                   629        597   -32    +5.1%
Coinbase                 243        233   -10    +4.1%
Firefox                  425        399   -26    +6.1%
Candy_Crush_Soda         552        538   -14    +2.5%
Hill_Climb_Racing        245        230   -15    +6.1%
Call_Recorder            437        426   -11    +2.5%
Color_Fill_3D            190        180   -10    +5.3%
eToro                    512        505    -7    +1.4%
GroupMe                  281        266   -15    +5.3%

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306010043.VJHcuCnb-lkp@intel.com/
Signed-off-by: Bongkyu Kim <bongkyu7.kim@...sung.com>
---
 .../admin-guide/kernel-parameters.txt         |   2 +
 kernel/locking/lock_events_list.h             |   5 +-
 kernel/locking/rwsem.c                        | 292 +++++++++++++++---
 3 files changed, 255 insertions(+), 44 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bb23a36a7ff7..b92a6b3f965f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5495,6 +5495,8 @@
 
 	rw		[KNL] Mount root device read-write on boot
 
+	rwsem.opt_rspin= [KNL] Use rwsem reader optimistic spinning
+
 	S		[KNL] Run init in single mode
 
 	s390_iommu=	[HW,S390]
diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
index 97fb6f3f840a..270a0d351932 100644
--- a/kernel/locking/lock_events_list.h
+++ b/kernel/locking/lock_events_list.h
@@ -56,9 +56,12 @@ LOCK_EVENT(rwsem_sleep_reader)	/* # of reader sleeps			*/
 LOCK_EVENT(rwsem_sleep_writer)	/* # of writer sleeps			*/
 LOCK_EVENT(rwsem_wake_reader)	/* # of reader wakeups			*/
 LOCK_EVENT(rwsem_wake_writer)	/* # of writer wakeups			*/
-LOCK_EVENT(rwsem_opt_lock)	/* # of opt-acquired write locks	*/
+LOCK_EVENT(rwsem_opt_rlock)	/* # of opt-acquired read locks		*/
+LOCK_EVENT(rwsem_opt_wlock)	/* # of opt-acquired write locks	*/
 LOCK_EVENT(rwsem_opt_fail)	/* # of failed optspins			*/
 LOCK_EVENT(rwsem_opt_nospin)	/* # of disabled optspins		*/
+LOCK_EVENT(rwsem_opt_norspin)	/* # of disabled reader-only optspins	*/
+LOCK_EVENT(rwsem_opt_rlock2)	/* # of opt-acquired 2ndary read locks	*/
 LOCK_EVENT(rwsem_rlock)		/* # of read locks acquired		*/
 LOCK_EVENT(rwsem_rlock_steal)	/* # of read locks by lock stealing	*/
 LOCK_EVENT(rwsem_rlock_fast)	/* # of fast read locks acquired	*/
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 9eabd585ce7a..03c03c86cd86 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -33,13 +33,19 @@
 #include "lock_events.h"
 
 /*
- * The least significant 2 bits of the owner value has the following
+ * The least significant 3 bits of the owner value has the following
  * meanings when set.
  *  - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
- *  - Bit 1: RWSEM_NONSPINNABLE - Cannot spin on a reader-owned lock
+ *  - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
+ *  - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.
  *
- * When the rwsem is reader-owned and a spinning writer has timed out,
- * the nonspinnable bit will be set to disable optimistic spinning.
+ * When the rwsem is either owned by an anonymous writer, or it is
+ * reader-owned, but a spinning writer has timed out, both nonspinnable
+ * bits will be set to disable optimistic spinning by readers and writers.
+ * In the later case, the last unlocking reader should then check the
+ * writer nonspinnable bit and clear it only to give writers preference
+ * to acquire the lock via optimistic spinning, but not readers. Similar
+ * action is also done in the reader slowpath.
 
  * When a writer acquires a rwsem, it puts its task_struct pointer
  * into the owner field. It is cleared after an unlock.
@@ -59,9 +65,47 @@
  * is previously owned by a writer and the following conditions are met:
  *  - rwsem is not currently writer owned
  *  - the handoff isn't set.
+ *
+ * Reader optimistic spinning is helpful when the reader critical section
+ * is short and there aren't that many readers around. It makes readers
+ * relatively more preferred than writers. When a writer times out spinning
+ * on a reader-owned lock and set the nospinnable bits, there are two main
+ * reasons for that.
+ *
+ *  1) The reader critical section is long, perhaps the task sleeps after
+ *     acquiring the read lock.
+ *  2) There are just too many readers contending the lock causing it to
+ *     take a while to service all of them.
+ *
+ * In the former case, long reader critical section will impede the progress
+ * of writers which is usually more important for system performance. In
+ * the later case, reader optimistic spinning tends to make the reader
+ * groups that contain readers that acquire the lock together smaller
+ * leading to more of them. That may hurt performance in some cases. In
+ * other words, the setting of nonspinnable bits indicates that reader
+ * optimistic spinning may not be helpful for those workloads that cause
+ * it.
+ *
+ * Therefore, any writers that had observed the setting of the writer
+ * nonspinnable bit for a given rwsem after they fail to acquire the lock
+ * via optimistic spinning will set the reader nonspinnable bit once they
+ * acquire the write lock. Similarly, readers that observe the setting
+ * of reader nonspinnable bit at slowpath entry will set the reader
+ * nonspinnable bits when they acquire the read lock via the wakeup path.
+ *
+ * Once the reader nonspinnable bit is on, it will only be reset when
+ * a writer is able to acquire the rwsem in the fast path or somehow a
+ * reader or writer in the slowpath doesn't observe the nonspinable bit.
+ *
+ * This is to discourage reader optmistic spinning on that particular
+ * rwsem and make writers more preferred. This adaptive disabling of reader
+ * optimistic spinning will alleviate the negative side effect of this
+ * feature.
  */
 #define RWSEM_READER_OWNED	(1UL << 0)
-#define RWSEM_NONSPINNABLE	(1UL << 1)
+#define RWSEM_RD_NONSPINNABLE	(1UL << 1)
+#define RWSEM_WR_NONSPINNABLE	(1UL << 2)
+#define RWSEM_NONSPINNABLE	(RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)
 #define RWSEM_OWNER_FLAGS_MASK	(RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
 
 #ifdef CONFIG_DEBUG_RWSEMS
@@ -127,6 +171,12 @@
 #define RWSEM_READ_FAILED_MASK	(RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
 				 RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
 
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+/* Reader optimistic spinning, default disabled */
+static bool rwsem_opt_rspin;
+module_param_named(opt_rspin, rwsem_opt_rspin, bool, 0644);
+#endif
+
 /*
  * All writes to owner are protected by WRITE_ONCE() to make sure that
  * store tearing can't happen as optimistic spinners may read and use
@@ -171,7 +221,7 @@ static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
 					    struct task_struct *owner)
 {
 	unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
-		(atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE);
+		(atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);
 
 	atomic_long_set(&sem->owner, val);
 }
@@ -341,6 +391,7 @@ struct rwsem_waiter {
 	enum rwsem_waiter_type type;
 	unsigned long timeout;
 	bool handoff_set;
+	unsigned long last_rowner;
 };
 #define rwsem_first_waiter(sem) \
 	list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
@@ -480,6 +531,10 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
 		 * the reader is copied over.
 		 */
 		owner = waiter->task;
+		if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
+			owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
+			lockevent_inc(rwsem_opt_norspin);
+		}
 		__rwsem_set_reader_owned(sem, owner);
 	}
 
@@ -684,6 +739,30 @@ enum owner_state {
 };
 
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+/*
+ * Try to acquire read lock before the reader is put on wait queue.
+ * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
+ * is ongoing.
+ */
+static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
+{
+	long count = atomic_long_read(&sem->count);
+
+	if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
+		return false;
+
+	count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
+	if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
+		rwsem_set_reader_owned(sem);
+		lockevent_inc(rwsem_opt_rlock);
+		return true;
+	}
+
+	/* Back out the change */
+	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
+	return false;
+}
+
 /*
  * Try to acquire write lock before the writer has been put on wait queue.
  */
@@ -695,14 +774,15 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 		if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
 					count | RWSEM_WRITER_LOCKED)) {
 			rwsem_set_owner(sem);
-			lockevent_inc(rwsem_opt_lock);
+			lockevent_inc(rwsem_opt_wlock);
 			return true;
 		}
 	}
 	return false;
 }
 
-static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
+					   unsigned long nonspinnable)
 {
 	struct task_struct *owner;
 	unsigned long flags;
@@ -721,7 +801,7 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 	/*
 	 * Don't check the read-owner as the entry may be stale.
 	 */
-	if ((flags & RWSEM_NONSPINNABLE) ||
+	if ((flags & nonspinnable) ||
 	    (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
 		ret = false;
 
@@ -732,9 +812,9 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 #define OWNER_SPINNABLE		(OWNER_NULL | OWNER_WRITER | OWNER_READER)
 
 static inline enum owner_state
-rwsem_owner_state(struct task_struct *owner, unsigned long flags)
+rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)
 {
-	if (flags & RWSEM_NONSPINNABLE)
+	if (flags & nonspinnable)
 		return OWNER_NONSPINNABLE;
 
 	if (flags & RWSEM_READER_OWNED)
@@ -744,7 +824,7 @@ rwsem_owner_state(struct task_struct *owner, unsigned long flags)
 }
 
 static noinline enum owner_state
-rwsem_spin_on_owner(struct rw_semaphore *sem)
+rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
 {
 	struct task_struct *new, *owner;
 	unsigned long flags, new_flags;
@@ -753,7 +833,7 @@ rwsem_spin_on_owner(struct rw_semaphore *sem)
 	lockdep_assert_preemption_disabled();
 
 	owner = rwsem_owner_flags(sem, &flags);
-	state = rwsem_owner_state(owner, flags);
+	state = rwsem_owner_state(owner, flags, nonspinnable);
 	if (state != OWNER_WRITER)
 		return state;
 
@@ -766,7 +846,7 @@ rwsem_spin_on_owner(struct rw_semaphore *sem)
 		 */
 		new = rwsem_owner_flags(sem, &new_flags);
 		if ((new != owner) || (new_flags != flags)) {
-			state = rwsem_owner_state(new, new_flags);
+			state = rwsem_owner_state(new, new_flags, nonspinnable);
 			break;
 		}
 
@@ -816,12 +896,14 @@ static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
 	return sched_clock() + delta;
 }
 
-static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
 {
 	bool taken = false;
 	int prev_owner_state = OWNER_NULL;
 	int loop = 0;
 	u64 rspin_threshold = 0;
+	unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
+					   : RWSEM_RD_NONSPINNABLE;
 
 	/* sem->wait_lock should not be held when doing optimistic spinning */
 	if (!osq_lock(&sem->osq))
@@ -836,14 +918,15 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 	for (;;) {
 		enum owner_state owner_state;
 
-		owner_state = rwsem_spin_on_owner(sem);
+		owner_state = rwsem_spin_on_owner(sem, nonspinnable);
 		if (!(owner_state & OWNER_SPINNABLE))
 			break;
 
 		/*
 		 * Try to acquire the lock
 		 */
-		taken = rwsem_try_write_lock_unqueued(sem);
+		taken = wlock ? rwsem_try_write_lock_unqueued(sem)
+			      : rwsem_try_read_lock_unqueued(sem);
 
 		if (taken)
 			break;
@@ -851,7 +934,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 		/*
 		 * Time-based reader-owned rwsem optimistic spinning
 		 */
-		if (owner_state == OWNER_READER) {
+		if (wlock && (owner_state == OWNER_READER)) {
 			/*
 			 * Re-initialize rspin_threshold every time when
 			 * the owner state changes from non-reader to reader.
@@ -860,7 +943,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 			 * the beginning of the 2nd reader phase.
 			 */
 			if (prev_owner_state != OWNER_READER) {
-				if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
+				if (rwsem_test_oflags(sem, nonspinnable))
 					break;
 				rspin_threshold = rwsem_rspin_threshold(sem);
 				loop = 0;
@@ -935,30 +1018,89 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 }
 
 /*
- * Clear the owner's RWSEM_NONSPINNABLE bit if it is set. This should
+ * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
  * only be called when the reader count reaches 0.
+ *
+ * This give writers better chance to acquire the rwsem first before
+ * readers when the rwsem was being held by readers for a relatively long
+ * period of time. Race can happen that an optimistic spinner may have
+ * just stolen the rwsem and set the owner, but just clearing the
+ * RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
  */
-static inline void clear_nonspinnable(struct rw_semaphore *sem)
+static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
 {
-	if (unlikely(rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)))
-		atomic_long_andnot(RWSEM_NONSPINNABLE, &sem->owner);
+	if (unlikely(rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE)))
+		atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
+}
+
+/*
+ * This function is called when the reader fails to acquire the lock via
+ * optimistic spinning. In this case we will still attempt to do a trylock
+ * when comparing the rwsem state right now with the state when entering
+ * the slowpath indicates that the reader is still in a valid reader phase.
+ * This happens when the following conditions are true:
+ *
+ * 1) The lock is currently reader owned, and
+ * 2) The lock is previously not reader-owned or the last read owner changes.
+ *
+ * In the former case, we have transitioned from a writer phase to a
+ * reader-phase while spinning. In the latter case, it means the reader
+ * phase hasn't ended when we entered the optimistic spinning loop. In
+ * both cases, the reader is eligible to acquire the lock. This is the
+ * secondary path where a read lock is acquired optimistically.
+ *
+ * The reader non-spinnable bit wasn't set at time of entry or it will
+ * not be here at all.
+ */
+static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
+					      unsigned long last_rowner)
+{
+	unsigned long owner = atomic_long_read(&sem->owner);
+
+	if (!(owner & RWSEM_READER_OWNED))
+		return false;
+
+	if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
+	    rwsem_try_read_lock_unqueued(sem)) {
+		lockevent_inc(rwsem_opt_rlock2);
+		lockevent_add(rwsem_opt_fail, -1);
+		return true;
+	}
+	return false;
+}
+
+static inline bool rwsem_no_spinners(struct rw_semaphore *sem)
+{
+	return !osq_is_locked(&sem->osq);
 }
 
 #else
-static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
+					   unsigned long nonspinnable)
 {
 	return false;
 }
 
-static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
 {
 	return false;
 }
 
-static inline void clear_nonspinnable(struct rw_semaphore *sem) { }
+static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }
+
+static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
+					      unsigned long last_rowner)
+{
+	return false;
+}
+
+static inline bool rwsem_no_spinners(struct rw_semaphore *sem)
+{
+	return false;
+}
 
 static inline enum owner_state
-rwsem_spin_on_owner(struct rw_semaphore *sem)
+rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
 {
 	return OWNER_NONSPINNABLE;
 }
@@ -984,7 +1126,7 @@ static inline void rwsem_cond_wake_waiter(struct rw_semaphore *sem, long count,
 		wake_type = RWSEM_WAKE_READERS;
 	} else {
 		wake_type = RWSEM_WAKE_ANY;
-		clear_nonspinnable(sem);
+		clear_wr_nonspinnable(sem);
 	}
 	rwsem_mark_wake(sem, wake_type, wake_q);
 }
@@ -995,32 +1137,66 @@ static inline void rwsem_cond_wake_waiter(struct rw_semaphore *sem, long count,
 static struct rw_semaphore __sched *
 rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int state)
 {
-	long adjustment = -RWSEM_READER_BIAS;
+	long owner, adjustment = -RWSEM_READER_BIAS;
 	long rcnt = (count >> RWSEM_READER_SHIFT);
 	struct rwsem_waiter waiter;
 	DEFINE_WAKE_Q(wake_q);
 
 	/*
 	 * To prevent a constant stream of readers from starving a sleeping
-	 * waiter, don't attempt optimistic lock stealing if the lock is
-	 * currently owned by readers.
+	 * waiter, don't attempt optimistic spinning if the lock is currently
+	 * owned by readers.
 	 */
-	if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
-	    (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
+	owner = atomic_long_read(&sem->owner);
+	if ((owner & RWSEM_READER_OWNED) && (rcnt > 1) &&
+	   !(count & RWSEM_WRITER_LOCKED))
 		goto queue;
 
 	/*
-	 * Reader optimistic lock stealing.
+	 * Reader optimistic lock stealing
+	 *
+	 * We can take the read lock directly without doing
+	 * rwsem_optimistic_spin() if the conditions are right.
+	 * Also wake up other readers if it is the first reader.
 	 */
-	if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
+	if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF)) &&
+	    rwsem_no_spinners(sem)) {
 		rwsem_set_reader_owned(sem);
 		lockevent_inc(rwsem_rlock_steal);
+		if (rcnt == 1)
+			goto wake_readers;
+		return sem;
+	}
+
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+	if (!rwsem_opt_rspin)
+		goto queue;
+#endif
 
+	/*
+	 * Save the current read-owner of rwsem, if available, and the
+	 * reader nonspinnable bit.
+	 */
+	waiter.last_rowner = owner;
+	if (!(waiter.last_rowner & RWSEM_READER_OWNED))
+		waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;
+
+	if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
+		goto queue;
+
+	/*
+	 * Undo read bias from down_read() and do optimistic spinning.
+	 */
+	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
+	adjustment = 0;
+	if (rwsem_optimistic_spin(sem, false)) {
+		/* rwsem_optimistic_spin() implies ACQUIRE on success */
 		/*
-		 * Wake up other readers in the wait queue if it is
-		 * the first reader.
+		 * Wake up other readers in the wait list if the front
+		 * waiter is a reader.
 		 */
-		if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
+wake_readers:
+		if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
 			raw_spin_lock_irq(&sem->wait_lock);
 			if (!list_empty(&sem->wait_list))
 				rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
@@ -1029,6 +1205,9 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 			wake_up_q(&wake_q);
 		}
 		return sem;
+	} else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
+		/* rwsem_reader_phase_trylock() implies ACQUIRE on success */
+		return sem;
 	}
 
 queue:
@@ -1045,7 +1224,8 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 		 * immediately as its RWSEM_READER_BIAS has already been set
 		 * in the count.
 		 */
-		if (!(atomic_long_read(&sem->count) & RWSEM_WRITER_MASK)) {
+		if (adjustment && !(atomic_long_read(&sem->count) &
+		     RWSEM_WRITER_MASK)) {
 			/* Provide lock ACQUIRE */
 			smp_acquire__after_ctrl_dep();
 			raw_spin_unlock_irq(&sem->wait_lock);
@@ -1058,7 +1238,10 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 	rwsem_add_waiter(sem, &waiter);
 
 	/* we're now waiting on the lock, but no longer actively locking */
-	count = atomic_long_add_return(adjustment, &sem->count);
+	if (adjustment)
+		count = atomic_long_add_return(adjustment, &sem->count);
+	else
+		count = atomic_long_read(&sem->count);
 
 	rwsem_cond_wake_waiter(sem, count, &wake_q);
 	raw_spin_unlock_irq(&sem->wait_lock);
@@ -1100,21 +1283,43 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 	return ERR_PTR(-EINTR);
 }
 
+/*
+ * This function is called by the a write lock owner. So the owner value
+ * won't get changed by others.
+ */
+static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
+						bool disable)
+{
+	if (unlikely(disable)) {
+		atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
+		lockevent_inc(rwsem_opt_norspin);
+	}
+}
+
 /*
  * Wait until we successfully acquire the write lock
  */
 static struct rw_semaphore __sched *
 rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 {
+	bool disable_rspin;
 	struct rwsem_waiter waiter;
 	DEFINE_WAKE_Q(wake_q);
 
 	/* do optimistic spinning and steal lock if possible */
-	if (rwsem_can_spin_on_owner(sem) && rwsem_optimistic_spin(sem)) {
+	if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
+	    rwsem_optimistic_spin(sem, true)) {
 		/* rwsem_optimistic_spin() implies ACQUIRE on success */
 		return sem;
 	}
 
+	/*
+	 * Disable reader optimistic spinning for this rwsem after
+	 * acquiring the write lock when the setting of the nonspinnable
+	 * bits are observed.
+	 */
+	disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;
+
 	/*
 	 * Optimistic spinning failed, proceed to the slowpath
 	 * and block until we can acquire the sem.
@@ -1170,7 +1375,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 		if (waiter.handoff_set) {
 			enum owner_state owner_state;
 
-			owner_state = rwsem_spin_on_owner(sem);
+			owner_state = rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE);
 			if (owner_state == OWNER_NULL)
 				goto trylock_again;
 		}
@@ -1182,6 +1387,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 		raw_spin_lock_irq(&sem->wait_lock);
 	}
 	__set_current_state(TASK_RUNNING);
+	rwsem_disable_reader_optspin(sem, disable_rspin);
 	raw_spin_unlock_irq(&sem->wait_lock);
 	lockevent_inc(rwsem_wlock);
 	trace_contention_end(sem, 0);
@@ -1348,7 +1554,7 @@ static inline void __up_read(struct rw_semaphore *sem)
 	DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
 	if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
 		      RWSEM_FLAG_WAITERS)) {
-		clear_nonspinnable(sem);
+		clear_wr_nonspinnable(sem);
 		rwsem_wake(sem);
 	}
 	preempt_enable();
-- 
2.36.1

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