[PATCH] locking/rwsem: Optionally re-enable reader optimistic spinning

From: Bongkyu Kim
Date: Tue May 30 2023 - 20:35:29 EST

Remove reader optimistic spinning has a great 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.

Test result:
This is 15 application startup performance in our s5e8535 soc.
- Cortex A78*2 + Cortex A55*6

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%

Signed-off-by: Bongkyu Kim <bongkyu7.kim@xxxxxxxxxxx>
---
.../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..016dbc4312e6 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(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