Re: [PATCH v5 00/18] locking/rwsem: Rwsem rearchitecture part 2

Ingo Molnar
2019-04-19T19:14:35Z

* Waiman Long <longman@redhat.com> wrote:

> On 04/18/2019 07:46 PM, Waiman Long wrote:
> > v5:
> > - Drop v4 patch 1 as it is merged into tip's locking/core branch.
> > - Integrate the 2 followup patches into the series. The first
> > follow-up patch is broken into 2 pieces. The first piece comes in
> > before the "Enable readers spinning on writer" and the 2nd piece
> > is merged into the "Enable time-based spinning on reader-owned
> > rwsem" patch. The 2nd followup patch is added after that.
> > - Add a new patch to make all wake_up_q() calls after dropping
> > wait_lock as suggested by PeterZ.
> > - Incorporate numerouos suggestions by PeterZ and Davidlohr.
>
> This patchset is still being reviewed by Peter . The purpose of this
> series is mainly to sync up the version that Peter has and the ones that
> I am working on incorporating his feedback. Further changes may still be
> needed.
>
> I run an overall performance test on this new patchset and present the
> data in this cover letter. However, I haven't run performance tests for
> individual patches. So the performance data listed in some of the
> patches may be stale.

Just for those who'd like to follow the scope of changes, find below the
v4->v5 interdiff. v5 is now included in tip:WIP.locking/core, and also
merged into tip:master. (But not propagated towards linux-next yet.)

Thanks,

Ingo

==============>
arch/x86/Kconfig | 6 +
include/linux/sched/wake_q.h | 5 +
kernel/Kconfig.locks | 12 +
kernel/locking/lock_events_list.h | 2 -
kernel/locking/rwsem.c | 538 +++++++++++++++++++-------------------
5 files changed, 291 insertions(+), 272 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 7d160f58a8f6..82a8c02f1b44 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -80,6 +80,7 @@ config X86
select ARCH_USE_BUILTIN_BSWAP
select ARCH_USE_QUEUED_RWLOCKS
select ARCH_USE_QUEUED_SPINLOCKS
+ select ARCH_USE_RWSEM_OWNER_COUNT if X86_64
select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
select ARCH_WANTS_DYNAMIC_TASK_STRUCT
select ARCH_WANTS_THP_SWAP if X86_64
@@ -350,6 +351,11 @@ config PGTABLE_LEVELS
default 3 if X86_PAE
default 2

+config RWSEM_OWNER_COUNT_PA_BITS
+ int
+ default 52 if X86_5LEVEL
+ default 46 if X86_64
+
config CC_HAS_SANE_STACKPROTECTOR
bool
default $(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC)) if 64BIT
diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
index ad826d2a4557..26a2013ac39c 100644
--- a/include/linux/sched/wake_q.h
+++ b/include/linux/sched/wake_q.h
@@ -51,6 +51,11 @@ static inline void wake_q_init(struct wake_q_head *head)
head->lastp = &head->first;
}

+static inline bool wake_q_empty(struct wake_q_head *head)
+{
+ return head->first == WAKE_Q_TAIL;
+}
+
extern void wake_q_add(struct wake_q_head *head, struct task_struct *task);
extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task);
extern void wake_up_q(struct wake_q_head *head);
diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
index e335953fa704..3370ea21407b 100644
--- a/kernel/Kconfig.locks
+++ b/kernel/Kconfig.locks
@@ -251,3 +251,15 @@ config ARCH_USE_QUEUED_RWLOCKS
config QUEUED_RWLOCKS
def_bool y if ARCH_USE_QUEUED_RWLOCKS
depends on SMP
+
+#
+# An architecture that want to merge rwsem write-owner into count should
+# select ARCH_USE_RWSEM_OWNER_COUNT and define RWSEM_OWNER_COUNT_PA_BITS
+# as the correct number of physical address bits.
+#
+config ARCH_USE_RWSEM_OWNER_COUNT
+ bool
+
+config RWSEM_OWNER_COUNT
+ def_bool y if ARCH_USE_RWSEM_OWNER_COUNT
+ depends on SMP && 64BIT
diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
index b0eeb77070dd..661326885800 100644
--- a/kernel/locking/lock_events_list.h
+++ b/kernel/locking/lock_events_list.h
@@ -65,8 +65,6 @@ LOCK_EVENT(rwsem_rlock) /* # of read locks acquired */
LOCK_EVENT(rwsem_rlock_fast) /* # of fast read locks acquired */
LOCK_EVENT(rwsem_rlock_fail) /* # of failed read lock acquisitions */
LOCK_EVENT(rwsem_rlock_handoff) /* # of read lock handoffs */
-LOCK_EVENT(rwsem_rtrylock) /* # of read trylock calls */
LOCK_EVENT(rwsem_wlock) /* # of write locks acquired */
LOCK_EVENT(rwsem_wlock_fail) /* # of failed write lock acquisitions */
LOCK_EVENT(rwsem_wlock_handoff) /* # of write lock handoffs */
-LOCK_EVENT(rwsem_wtrylock) /* # of write trylock calls */
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index d50bc7b0315f..19d8fbd50d17 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -10,8 +10,9 @@
* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
*
- * Rwsem count bit fields re-definition and rwsem rearchitecture
- * by Waiman Long <longman@redhat.com>.
+ * Rwsem count bit fields re-definition and rwsem rearchitecture by
+ * Waiman Long <longman@redhat.com> and
+ * Peter Zijlstra <peterz@infradead.org>.
*/

#include <linux/types.h>
@@ -44,10 +45,9 @@
* 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 or the count itself (64-bit only. It should
- * be cleared after an unlock.
+ * into the owner field. It is cleared after an unlock.
*
* When a reader acquires a rwsem, it will also puts its task_struct
* pointer into the owner field with the RWSEM_READER_OWNED bit set.
@@ -107,13 +107,6 @@
# define DEBUG_RWSEMS_WARN_ON(c, sem)
#endif

-/*
- * Enable the merging of owner into count for x86-64 only.
- */
-#ifdef CONFIG_X86_64
-#define RWSEM_MERGE_OWNER_TO_COUNT
-#endif
-
/*
* With separate count and owner, there are timing windows where the two
* values are inconsistent. That can cause problem when trying to figure
@@ -127,14 +120,17 @@
* architectures, the long integer value just isn't big enough for
* combining owner and count. So they remain separate.
*
- * For x86-64, the physical address can use up to 52 bits. That is 4PB
- * of memory. That leaves 12 bits available for other use. The task
- * structure pointer is also aligned to the L1 cache size. That means
- * another 6 bits (64 bytes cacheline) will be available. Reserving
- * 2 bits for status flags, we will have 16 bits for the reader count
- * and read fail bit. That can supports up to (32k-1) active readers.
+ * For x86-64, the physical address can use up to 52 bits if
+ * CONFIG_X86_5LEVEL. That is 4PB of memory. That leaves 12 bits
+ * available for other use. The task structure pointer is also aligned
+ * to the L1 cache size. That means another 6 bits (64 bytes cacheline)
+ * will be available. Reserving 2 bits for status flags, we will have
+ * 16 bits for the reader count and read fail bit. That can supports up
+ * to (32k-1) active readers. If 5-level page table support isn't
+ * configured, we can supports up to (2M-1) active readers.
*
- * On x86-64, the bit definitions of the count are:
+ * On x86-64 with CONFIG_X86_5LEVEL and CONFIG_RWSEM_OWNER_COUNT, the bit
+ * definitions of the count are:
*
* Bit 0 - waiters present bit
* Bit 1 - lock handoff bit
@@ -142,7 +138,8 @@
* Bits 48-62 - 15-bit reader counts
* Bit 63 - read fail bit
*
- * On other 64-bit architectures, the bit definitions are:
+ * On other 64-bit architectures without CONFIG_RWSEM_OWNER_COUNT, the bit
+ * definitions are:
*
* Bit 0 - writer locked bit
* Bit 1 - waiters present bit
@@ -181,23 +178,16 @@
#define RWSEM_FLAG_HANDOFF (1UL << 1)
#define RWSEM_FLAG_READFAIL (1UL << (BITS_PER_LONG - 1))

-
-#ifdef RWSEM_MERGE_OWNER_TO_COUNT
-
-#ifdef __PHYSICAL_MASK_SHIFT
-#define RWSEM_PA_MASK_SHIFT __PHYSICAL_MASK_SHIFT
-#else
-#define RWSEM_PA_MASK_SHIFT 52
-#endif
-#define RWSEM_READER_SHIFT (RWSEM_PA_MASK_SHIFT - L1_CACHE_SHIFT + 2)
+#ifdef CONFIG_RWSEM_OWNER_COUNT
+#define RWSEM_READER_SHIFT (CONFIG_RWSEM_OWNER_COUNT_PA_BITS -\
+ L1_CACHE_SHIFT + 2)
#define RWSEM_WRITER_MASK ((1UL << RWSEM_READER_SHIFT) - 4)
#define RWSEM_WRITER_LOCKED rwsem_owner_count(current)
-
-#else /* RWSEM_MERGE_OWNER_TO_COUNT */
+#else /* !CONFIG_RWSEM_OWNER_COUNT */
#define RWSEM_READER_SHIFT 8
#define RWSEM_WRITER_MASK (1UL << 7)
#define RWSEM_WRITER_LOCKED RWSEM_WRITER_MASK
-#endif /* RWSEM_MERGE_OWNER_TO_COUNT */
+#endif /* CONFIG_RWSEM_OWNER_COUNT */

#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
@@ -205,14 +195,6 @@
#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)

-#define RWSEM_COUNT_LOCKED(c) ((c) & RWSEM_LOCK_MASK)
-#define RWSEM_COUNT_WLOCKED(c) ((c) & RWSEM_WRITER_MASK)
-#define RWSEM_COUNT_HANDOFF(c) ((c) & RWSEM_FLAG_HANDOFF)
-#define RWSEM_COUNT_LOCKED_OR_HANDOFF(c) \
- ((c) & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))
-#define RWSEM_COUNT_WLOCKED_OR_HANDOFF(c) \
- ((c) & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
-
/*
* Task structure pointer compression (64-bit only):
* (owner - PAGE_OFFSET) >> (L1_CACHE_SHIFT - 2)
@@ -251,10 +233,10 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
WRITE_ONCE(sem->owner, NULL);
}

-#ifdef RWSEM_MERGE_OWNER_TO_COUNT
+#ifdef CONFIG_RWSEM_OWNER_COUNT
/*
* Get the owner value from count to have early access to the task structure.
- * Owner from sem->count should includes the RWSEM_NONSPINNABLE bit
+ * Owner from sem->count should includes the RWSEM_NONSPINNABLE bits
* from sem->owner.
*/
static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
@@ -265,16 +247,12 @@ static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
return (struct task_struct *) (cowner
? cowner | (sowner & RWSEM_NONSPINNABLE) : sowner);
}
-#else /* !RWSEM_MERGE_OWNER_TO_COUNT */
+#else /* !CONFIG_RWSEM_OWNER_COUNT */
static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
{
- unsigned long owner = (unsigned long)READ_ONCE(sem->owner);
-
- /* Clear all the flag bits for writer */
- return (struct task_struct *)((owner & RWSEM_READER_OWNED)
- ? owner : (owner & ~RWSEM_OWNER_FLAGS_MASK));
+ return READ_ONCE(sem->owner);
}
-#endif /* RWSEM_MERGE_OWNER_TO_COUNT */
+#endif /* CONFIG_RWSEM_OWNER_COUNT */

/*
* The task_struct pointer of the last owning reader will be left in
@@ -302,18 +280,13 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
* and steal the lock.
* N.B. !owner is considered spinnable.
*/
-static inline bool is_rwsem_owner_spinnable(void *owner, bool wr)
+static inline bool is_rwsem_owner_spinnable(struct task_struct *owner, bool wr)
{
unsigned long bit = wr ? RWSEM_WR_NONSPINNABLE : RWSEM_RD_NONSPINNABLE;

return !((unsigned long)owner & bit);
}

-static inline bool is_rwsem_owner_reader(void *owner)
-{
- return (unsigned long)owner & RWSEM_READER_OWNED;
-}
-
/*
* Return true if the rwsem is spinnable.
*/
@@ -368,8 +341,8 @@ static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
{
long owner = (long)READ_ONCE(sem->owner);

- while (is_rwsem_owner_reader((struct task_struct *)owner)) {
- if (!is_rwsem_owner_spinnable((void *)owner, true))
+ while (owner & RWSEM_READER_OWNED) {
+ if (!is_rwsem_owner_spinnable((void *)owner, false))
break;
owner = cmpxchg((long *)&sem->owner, owner,
owner | RWSEM_NONSPINNABLE);
@@ -400,9 +373,9 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
struct lock_class_key *key)
{
/*
- * We should support at least (4k-1) concurrent readers
+ * We should support at least (8k-1) concurrent readers
*/
- BUILD_BUG_ON(sizeof(long) * 8 - RWSEM_READER_SHIFT < 12);
+ BUILD_BUG_ON(sizeof(long) * 8 - RWSEM_READER_SHIFT < 14);

#ifdef CONFIG_DEBUG_LOCK_ALLOC
/*
@@ -432,6 +405,8 @@ struct rwsem_waiter {
enum rwsem_waiter_type type;
unsigned long timeout;
};
+#define rwsem_first_waiter(sem) \
+ list_first_entry(&sem->wait_list, struct rwsem_waiter, list)

enum rwsem_wake_type {
RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
@@ -447,15 +422,16 @@ enum writer_wait_state {

/*
* The typical HZ value is either 250 or 1000. So set the minimum waiting
- * time to 4ms in the wait queue before initiating the handoff protocol.
+ * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
+ * queue before initiating the handoff protocol.
*/
-#define RWSEM_WAIT_TIMEOUT (HZ/250)
+#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)

/*
- * We limit the maximum number of readers that can be woken up for a
- * wake-up call to not penalizing the waking thread for spending too
- * much time doing it as well as the unlikely possiblity of overflowing
- * the reader count.
+ * Magic number to batch-wakeup waiting readers, even when writers are
+ * also present in the queue. This both limits the amount of work the
+ * waking thread must do and also prevents any potential counter overflow,
+ * however unlikely.
*/
#define MAX_READERS_WAKEUP 0x100

@@ -484,7 +460,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
* Take a peek at the queue head waiter such that we can determine
* the wakeup(s) to perform.
*/
- waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
+ waiter = rwsem_first_waiter(sem);

if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
if (wake_type == RWSEM_WAKE_ANY) {
@@ -512,7 +488,9 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
oldcount = atomic_long_fetch_add(adjustment, &sem->count);
if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
/*
- * Initiate handoff to reader, if applicable.
+ * When we've been waiting "too" long (for writers
+ * to give up the lock), request a * HANDOFF to
+ * force the issue.
*/
if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
time_after(jiffies, waiter->timeout)) {
@@ -520,7 +498,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
lockevent_inc(rwsem_rlock_handoff);
}

- atomic_long_sub(adjustment, &sem->count);
+ atomic_long_add(-adjustment, &sem->count);
return;
}
/*
@@ -532,9 +510,15 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,

/*
* Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
- * queue. We know that woken will be at least 1 as we accounted for
- * above. Note we increment the 'active part' of the count by the
+ * queue. We know that the woken will be at least 1 as we accounted
+ * for above. Note we increment the 'active part' of the count by the
* number of readers before waking any processes up.
+ *
+ * This is an adaptation of the phase-fair R/W locks where at the
+ * reader phase (first waiter is a reader), all readers are eligible
+ * to acquire the lock at the same time irrespective of their order
+ * in the queue. The writers acquire the lock according to their
+ * order in the queue.
*/
list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
struct task_struct *tsk;
@@ -549,7 +533,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
list_del(&waiter->list);
/*
* Ensure calling get_task_struct() before setting the reader
- * waiter to nil such that rwsem_down_read_failed() cannot
+ * waiter to nil such that rwsem_down_read_slowpath() cannot
* race with do_exit() by always holding a reference count
* to the task to wakeup.
*/
@@ -575,9 +559,10 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
}

/*
- * Clear the handoff flag
+ * When we've woken a reader, we no longer need to force writers
+ * to give up the lock and we can clear HANDOFF.
*/
- if (woken && RWSEM_COUNT_HANDOFF(atomic_long_read(&sem->count)))
+ if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
adjustment -= RWSEM_FLAG_HANDOFF;

if (adjustment)
@@ -590,7 +575,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
* sem->count accordingly.
*
* If wstate is WRITER_HANDOFF, it will make sure that either the handoff
- * bit is set or the lock is acquired.
+ * bit is set or the lock is acquired with handoff bit cleared.
*/
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem,
const long wlock,
@@ -598,34 +583,34 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem,
{
long new;

-retry:
- if (RWSEM_COUNT_LOCKED(count)) {
- if (RWSEM_COUNT_HANDOFF(count) || (wstate != WRITER_HANDOFF))
- return false;
- /*
- * The lock may become free just before setting handoff bit.
- * It will be simpler if atomic_long_or_return() is available.
- */
- atomic_long_or(RWSEM_FLAG_HANDOFF, &sem->count);
- count = atomic_long_read(&sem->count);
- goto retry;
- }
+ lockdep_assert_held(&sem->wait_lock);
+ do {
+ bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);

- if ((wstate == WRITER_NOT_FIRST) && RWSEM_COUNT_HANDOFF(count))
- return false;
+ if (has_handoff && wstate == WRITER_NOT_FIRST)
+ return false;

- new = (count & ~RWSEM_FLAG_HANDOFF) + wlock -
- (list_is_singular(&sem->wait_list) ? RWSEM_FLAG_WAITERS : 0);
+ if (count & RWSEM_LOCK_MASK) {
+ if (has_handoff || (wstate != WRITER_HANDOFF))
+ return false;
+ new = count | RWSEM_FLAG_HANDOFF;
+ } else {
+ new = (count | wlock) & ~RWSEM_FLAG_HANDOFF;

- if (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new)) {
- rwsem_set_owner(sem);
- return true;
- }
+ if (list_is_singular(&sem->wait_list))
+ new &= ~RWSEM_FLAG_WAITERS;
+ }
+ } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));

- if (unlikely((wstate == WRITER_HANDOFF) && !RWSEM_COUNT_HANDOFF(count)))
- goto retry;
+ /*
+ * We have either acquired the lock with handoff bit cleared or
+ * set the handoff bit.
+ */
+ if (new & RWSEM_FLAG_HANDOFF)
+ return false;

- return false;
+ rwsem_set_owner(sem);
+ return true;
}

#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
@@ -638,11 +623,11 @@ static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
{
long count = atomic_long_read(&sem->count);

- if (RWSEM_COUNT_WLOCKED_OR_HANDOFF(count))
+ if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
return false;

count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
- if (!RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) {
+ if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
rwsem_set_reader_owned(sem);
lockevent_inc(rwsem_opt_rlock);
return true;
@@ -661,9 +646,9 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem,
{
long count = atomic_long_read(&sem->count);

- while (!RWSEM_COUNT_LOCKED_OR_HANDOFF(count)) {
+ while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
- count + wlock)) {
+ count | wlock)) {
rwsem_set_owner(sem);
lockevent_inc(rwsem_opt_wlock);
return true;
@@ -674,6 +659,11 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem,

static inline bool owner_on_cpu(struct task_struct *owner)
{
+ /*
+ * Clear all the flag bits in owner
+ */
+ *((unsigned long *)&owner) &= ~RWSEM_OWNER_FLAGS_MASK;
+
/*
* As lock holder preemption issue, we both skip spinning if
* task is not on cpu or its cpu is preempted
@@ -698,7 +688,8 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem, bool wr)
owner = rwsem_get_owner(sem);
if (owner) {
ret = is_rwsem_owner_spinnable(owner, wr) &&
- (is_rwsem_owner_reader(owner) || owner_on_cpu(owner));
+ (((unsigned long)owner & RWSEM_READER_OWNED) ||
+ owner_on_cpu(owner));
}
rcu_read_unlock();
preempt_enable();
@@ -708,7 +699,8 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem, bool wr)
}

/*
- * Return the folowing 4 values depending on the lock owner state.
+ * The rwsem_spin_on_owner() function returns the folowing 4 values
+ * depending on the lock owner state.
* OWNER_NULL : owner is currently NULL
* OWNER_WRITER: when owner changes and is a writer
* OWNER_READER: when owner changes and the new owner may be a reader.
@@ -725,22 +717,40 @@ enum owner_state {
};
#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER)

+static inline enum owner_state rwsem_owner_state(unsigned long owner, bool wr)
+{
+ if (!owner)
+ return OWNER_NULL;
+
+ if (!is_rwsem_owner_spinnable((void *)owner, wr))
+ return OWNER_NONSPINNABLE;
+
+ if (owner & RWSEM_READER_OWNED)
+ return OWNER_READER;
+
+ return OWNER_WRITER;
+}
+
static noinline enum owner_state
rwsem_spin_on_owner(struct rw_semaphore *sem, bool wr)
{
- struct task_struct *owner = rwsem_get_owner(sem);
- long count;
+ struct task_struct *tmp, *owner = rwsem_get_owner(sem);
+ enum owner_state state = rwsem_owner_state((unsigned long)owner, wr);

- if (!is_rwsem_owner_spinnable(owner, wr))
- return OWNER_NONSPINNABLE;
+ if (state != OWNER_WRITER)
+ return state;

rcu_read_lock();
- while (owner && !is_rwsem_owner_reader(owner)) {
- struct task_struct *new_owner = rwsem_get_owner(sem);
+ for (;;) {
+ if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) {
+ state = OWNER_NONSPINNABLE;
+ break;
+ }

- if (new_owner != owner) {
- owner = new_owner;
- break; /* The owner has changed */
+ tmp = rwsem_get_owner(sem);
+ if (tmp != owner) {
+ state = rwsem_owner_state((unsigned long)tmp, wr);
+ break;
}

/*
@@ -751,33 +761,16 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, bool wr)
*/
barrier();

- /*
- * abort spinning when need_resched or owner is not running or
- * owner's cpu is preempted.
- */
if (need_resched() || !owner_on_cpu(owner)) {
- rcu_read_unlock();
- return OWNER_NONSPINNABLE;
+ state = OWNER_NONSPINNABLE;
+ break;
}

cpu_relax();
}
rcu_read_unlock();

- /*
- * If there is a new owner or the owner is not set, we continue
- * spinning except when here is no active locks and the handoff bit
- * is set. In this case, we have to stop spinning.
- */
- if (!is_rwsem_owner_spinnable(owner, wr))
- return OWNER_NONSPINNABLE;
- if (owner && !is_rwsem_owner_reader(owner))
- return OWNER_WRITER;
-
- count = atomic_long_read(&sem->count);
- if (RWSEM_COUNT_HANDOFF(count) && !RWSEM_COUNT_LOCKED(count))
- return OWNER_NONSPINNABLE;
- return !owner ? OWNER_NULL : OWNER_READER;
+ return state;
}

/*
@@ -795,8 +788,11 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, bool wr)
*
* In the first case when RWSEM_FLAG_WAITERS is set, no new reader can
* become rwsem owner. It is assumed that the more readers own the rwsem,
- * the longer it will take for them to wind down and free the rwsem. This
- * is subjected to a maximum value of 25us.
+ * the longer it will take for them to wind down and free the rwsem. In
+ * addition, if it happens that a previous task that releases the lock
+ * is in the process of waking up readers one-by-one, the process will
+ * take longer when more readers needed to be woken up. This is subjected
+ * to a maximum value of 25us.
*
* In the second case with RWSEM_FLAG_WAITERS off, new readers can join
* and become one of the owners. So assuming for the worst case and spin
@@ -805,19 +801,22 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, bool wr)
static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
{
long count = atomic_long_read(&sem->count);
- int reader_cnt = atomic_long_read(&sem->count) >> RWSEM_READER_SHIFT;
+ u64 delta = 25 * NSEC_PER_USEC;
+
+ if (count & RWSEM_FLAG_WAITERS) {
+ int readers = count >> RWSEM_READER_SHIFT;

- if (reader_cnt > 30)
- reader_cnt = 30;
- return sched_clock() + ((count & RWSEM_FLAG_WAITERS)
- ? 10 * NSEC_PER_USEC + reader_cnt * NSEC_PER_USEC/2
- : 25 * NSEC_PER_USEC);
+ if (readers > 30)
+ readers = 30;
+ delta = (20 + readers) * NSEC_PER_USEC / 2;
+ }
+
+ return sched_clock() + delta;
}

static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
{
bool taken = false;
- bool is_rt_task = rt_task(current);
int prev_owner_state = OWNER_NULL;
int loop = 0;
u64 rspin_threshold = 0;
@@ -832,7 +831,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
* Optimistically spin on the owner field and attempt to acquire the
* lock whenever the owner changes. Spinning will be stopped when:
* 1) the owning writer isn't running; or
- * 2) readers own the lock and spinning count has reached 0.
+ * 2) readers own the lock and spinning time has exceeded limit.
*/
for (;;) {
enum owner_state owner_state = rwsem_spin_on_owner(sem, wlock);
@@ -854,8 +853,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
*/
if (wlock && (owner_state == OWNER_READER)) {
/*
- * Initialize rspin_threshold when the owner
- * state changes from non-reader to reader.
+ * Re-initialize rspin_threshold every time when
+ * the owner state changes from non-reader to reader.
+ * This allows a writer to steal the lock in between
+ * 2 reader phases and have the threshold reset at
+ * the beginning of the 2nd reader phase.
*/
if (prev_owner_state != OWNER_READER) {
if (!is_rwsem_spinnable(sem, wlock))
@@ -865,10 +867,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
}

/*
- * Check time threshold every 16 iterations to
- * avoid calling sched_clock() too frequently.
- * This will make the actual spinning time a
- * bit more than that specified in the threshold.
+ * Check time threshold once every 16 iterations to
+ * avoid calling sched_clock() too frequently so
+ * as to reduce the average latency between the times
+ * when the lock becomes free and when the spinner
+ * is ready to do a trylock.
*/
else if (!(++loop & 0xf) &&
(sched_clock() > rspin_threshold)) {
@@ -882,13 +885,28 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
* An RT task cannot do optimistic spinning if it cannot
* be sure the lock holder is running or live-lock may
* happen if the current task and the lock holder happen
- * to run in the same CPU.
+ * to run in the same CPU. However, aborting optimistic
+ * spinning while a NULL owner is detected may miss some
+ * opportunity where spinning can continue without causing
+ * problem.
+ *
+ * There are 2 possible cases where an RT task may be able
+ * to continue spinning.
*
- * When there's no owner or is reader-owned, an RT task
- * will stop spinning if the owner state is not a writer
- * at the previous iteration of the loop. This allows the
- * RT task to recheck if the task that steals the lock is
- * a spinnable writer. If so, it can keeps on spinning.
+ * 1) The lock owner is in the process of releasing the
+ * lock, sem->owner is cleared but the lock has not
+ * been released yet.
+ * 2) The lock was free and owner cleared, but another
+ * task just comes in and acquire the lock before
+ * we try to get it. The new owner may be a spinnable
+ * writer.
+ *
+ * To take advantage of two scenarios listed agove, the RT
+ * task is made to retry one more time to see if it can
+ * acquire the lock or continue spinning on the new owning
+ * writer. Of course, if the time lag is long enough or the
+ * new owner is not a writer or spinnable, the RT task will
+ * quit spinning.
*
* If the owner is a writer, the need_resched() check is
* done inside rwsem_spin_on_owner(). If the owner is not
@@ -897,7 +915,8 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
if (owner_state != OWNER_WRITER) {
if (need_resched())
break;
- if (is_rt_task && (prev_owner_state != OWNER_WRITER))
+ if (rt_task(current) &&
+ (prev_owner_state != OWNER_WRITER))
break;
}
prev_owner_state = owner_state;
@@ -916,6 +935,23 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
lockevent_cond_inc(rwsem_opt_fail, !taken);
return taken;
}
+
+/*
+ * 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_wr_nonspinnable(struct rw_semaphore *sem)
+{
+ if (!is_rwsem_spinnable(sem, true))
+ atomic_long_andnot(RWSEM_WR_NONSPINNABLE,
+ (atomic_long_t *)&sem->owner);
+}
#else
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem, bool wr)
{
@@ -927,25 +963,18 @@ static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem,
{
return false;
}
-#endif

-/*
- * This is safe to be called without holding the wait_lock.
- */
-static inline bool
-rwsem_waiter_is_first(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
-{
- return list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
- == waiter;
-}
+static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }
+#endif

/*
* Wait for the read lock to be granted
*/
-static inline struct rw_semaphore __sched *
-__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state, long count)
+static struct rw_semaphore __sched *
+rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count)
{
long adjustment = -RWSEM_READER_BIAS;
+ bool wake = false;
struct rwsem_waiter waiter;
DEFINE_WAKE_Q(wake_q);

@@ -957,7 +986,7 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state, long count)
* reader count.
*
* As preemption is not disabled, there is a remote
- * possibility that premption can happen in the narrow
+ * possibility that preemption can happen in the narrow
* timing window between incrementing and decrementing
* the reader count and the task is put to sleep for a
* considerable amount of time. If sufficient number
@@ -980,19 +1009,17 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state, long count)
*/
atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
adjustment = 0;
- if (rwsem_optimistic_spin(sem, 0)) {
- unsigned long flags;
-
+ if (rwsem_optimistic_spin(sem, false)) {
/*
* Opportunistically wake up other readers in the wait queue.
* It has another chance of wakeup at unlock time.
*/
if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS) &&
- raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
+ raw_spin_trylock_irq(&sem->wait_lock)) {
if (!list_empty(&sem->wait_list))
__rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
&wake_q);
- raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+ raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
}
return sem;
@@ -1030,21 +1057,17 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state, long count)

/*
* If there are no active locks, wake the front queued process(es).
- * Also clear the owner's RWSEM_WR_NONSPINNABLE bit if set.
*
* If there are no writers and we are first in the queue,
* wake our own waiter to join the existing active readers !
*/
- if (!RWSEM_COUNT_LOCKED(count)) {
- /* Clear RWSEM_WR_UNSPINNABLE bit if set */
- if (!is_rwsem_spinnable(sem, true))
- atomic_long_andnot(RWSEM_WR_NONSPINNABLE,
- (atomic_long_t *)&sem->owner);
- __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
- } else if (!(count & RWSEM_WRITER_MASK) &&
- (adjustment & RWSEM_FLAG_WAITERS)) {
- __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
+ if (!(count & RWSEM_LOCK_MASK)) {
+ clear_wr_nonspinnable(sem);
+ wake = true;
}
+ if (wake || (!(count & RWSEM_WRITER_MASK) &&
+ (adjustment & RWSEM_FLAG_WAITERS)))
+ __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
@@ -1070,27 +1093,16 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state, long count)
return sem;
out_nolock:
list_del(&waiter.list);
- if (list_empty(&sem->wait_list))
+ if (list_empty(&sem->wait_list)) {
atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
&sem->count);
+ }
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
lockevent_inc(rwsem_rlock_fail);
return ERR_PTR(-EINTR);
}

-static inline struct rw_semaphore * __sched
-rwsem_down_read_failed(struct rw_semaphore *sem, long cnt)
-{
- return __rwsem_down_read_failed_common(sem, TASK_UNINTERRUPTIBLE, cnt);
-}
-
-static inline struct rw_semaphore * __sched
-rwsem_down_read_failed_killable(struct rw_semaphore *sem, long cnt)
-{
- return __rwsem_down_read_failed_common(sem, TASK_KILLABLE, cnt);
-}
-
static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
bool disable)
{
@@ -1103,16 +1115,16 @@ static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
/*
* Wait until we successfully acquire the write lock
*/
-static inline struct rw_semaphore *
-__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
+static struct rw_semaphore *
+rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
{
long count;
+ bool disable_rspin;
enum writer_wait_state wstate;
struct rwsem_waiter waiter;
struct rw_semaphore *ret = sem;
DEFINE_WAKE_Q(wake_q);
const long wlock = RWSEM_WRITER_LOCKED;
- bool disable_rspin;

/* do optimistic spinning and steal lock if possible */
if (rwsem_can_spin_on_owner(sem, true) &&
@@ -1153,27 +1165,29 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
* must be read owned; so we try to wake any read lock
* waiters that were queued ahead of us.
*/
- if (!RWSEM_COUNT_LOCKED(count))
- __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
- else if (!(count & RWSEM_WRITER_MASK) &&
- (count & RWSEM_READER_MASK))
- __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
- else
+ if (count & RWSEM_WRITER_MASK)
goto wait;

- /*
- * The wakeup is normally called _after_ the wait_lock
- * is released, but given that we are proactively waking
- * readers we can deal with the wake_q overhead as it is
- * similar to releasing and taking the wait_lock again
- * for attempting rwsem_try_write_lock().
- */
- wake_up_q(&wake_q);
+ __rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
+ ? RWSEM_WAKE_READERS
+ : RWSEM_WAKE_ANY, &wake_q);

- /*
- * Reinitialize wake_q after use.
- */
- wake_q_init(&wake_q);
+ if (!wake_q_empty(&wake_q)) {
+ /*
+ * We want to minimize wait_lock hold time especially
+ * when a large number of readers are to be woken up.
+ */
+ raw_spin_unlock_irq(&sem->wait_lock);
+ wake_up_q(&wake_q);
+ wake_q_init(&wake_q); /* Used again, reinit */
+ raw_spin_lock_irq(&sem->wait_lock);
+ /*
+ * This waiter may have become first in the wait
+ * list after re-acquring the wait_lock. The
+ * rwsem_first_waiter() test in the main while
+ * loop below will correctly detect that.
+ */
+ }
} else {
count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count);
}
@@ -1195,19 +1209,22 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
schedule();
lockevent_inc(rwsem_sleep_writer);
set_current_state(state);
- count = atomic_long_read(&sem->count);
+ /*
+ * If HANDOFF bit is set, unconditionally do
+ * a trylock.
+ */
+ if (wstate == WRITER_HANDOFF)
+ break;

if ((wstate == WRITER_NOT_FIRST) &&
- rwsem_waiter_is_first(sem, &waiter))
+ (rwsem_first_waiter(sem) == &waiter))
wstate = WRITER_FIRST;

- if (!RWSEM_COUNT_LOCKED(count))
+ count = atomic_long_read(&sem->count);
+ if (!(count & RWSEM_LOCK_MASK))
break;

/*
- * An RT task sets the HANDOFF bit immediately.
- * Non-RT task will wait a while before doing so.
- *
* The setting of the handoff bit is deferred
* until rwsem_try_write_lock() is called.
*/
@@ -1215,9 +1232,6 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
time_after(jiffies, waiter.timeout))) {
wstate = WRITER_HANDOFF;
lockevent_inc(rwsem_wlock_handoff);
- /*
- * Break out to call rwsem_try_write_lock().
- */
break;
}
}
@@ -1237,12 +1251,10 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
__set_current_state(TASK_RUNNING);
raw_spin_lock_irq(&sem->wait_lock);
list_del(&waiter.list);
- /*
- * If handoff bit has been set by this waiter, make sure that the
- * clearing of it is seen by others before proceeding.
- */
+
if (unlikely(wstate == WRITER_HANDOFF))
- atomic_long_add_return(-RWSEM_FLAG_HANDOFF, &sem->count);
+ atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count);
+
if (list_empty(&sem->wait_list))
atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
else
@@ -1254,18 +1266,6 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
return ERR_PTR(-EINTR);
}

-static inline struct rw_semaphore * __sched
-rwsem_down_write_failed(struct rw_semaphore *sem)
-{
- return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
-}
-
-static inline struct rw_semaphore * __sched
-rwsem_down_write_failed_killable(struct rw_semaphore *sem)
-{
- return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
-}
-
/*
* handle waking up a waiter on the semaphore
* - up_read/up_write has decremented the active part of count if we come here
@@ -1312,11 +1312,11 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
*/
inline void __down_read(struct rw_semaphore *sem)
{
- long count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
- &sem->count);
+ long tmp = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
+ &sem->count);

- if (unlikely(count & RWSEM_READ_FAILED_MASK)) {
- rwsem_down_read_failed(sem, count);
+ if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) {
+ rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, tmp);
DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
} else {
rwsem_set_reader_owned(sem);
@@ -1325,11 +1325,11 @@ inline void __down_read(struct rw_semaphore *sem)

static inline int __down_read_killable(struct rw_semaphore *sem)
{
- long count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
- &sem->count);
+ long tmp = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
+ &sem->count);

- if (unlikely(count & RWSEM_READ_FAILED_MASK)) {
- if (IS_ERR(rwsem_down_read_failed_killable(sem, count)))
+ if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) {
+ if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE, tmp)))
return -EINTR;
DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
} else {
@@ -1345,7 +1345,6 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
*/
long tmp = RWSEM_UNLOCKED_VALUE;

- lockevent_inc(rwsem_rtrylock);
do {
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
tmp + RWSEM_READER_BIAS)) {
@@ -1361,21 +1360,25 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
*/
static inline void __down_write(struct rw_semaphore *sem)
{
- if (unlikely(atomic_long_cmpxchg_acquire(&sem->count, 0,
- RWSEM_WRITER_LOCKED)))
- rwsem_down_write_failed(sem);
+ long tmp = RWSEM_UNLOCKED_VALUE;
+
+ if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
+ RWSEM_WRITER_LOCKED)))
+ rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
else
rwsem_set_owner(sem);
-#ifdef RWSEM_MERGE_OWNER_TO_COUNT
+#ifdef CONFIG_RWSEM_OWNER_COUNT
DEBUG_RWSEMS_WARN_ON(sem->owner != rwsem_get_owner(sem), sem);
#endif
}

static inline int __down_write_killable(struct rw_semaphore *sem)
{
- if (unlikely(atomic_long_cmpxchg_acquire(&sem->count, 0,
- RWSEM_WRITER_LOCKED))) {
- if (IS_ERR(rwsem_down_write_failed_killable(sem)))
+ long tmp = RWSEM_UNLOCKED_VALUE;
+
+ if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
+ RWSEM_WRITER_LOCKED))) {
+ if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
return -EINTR;
} else {
rwsem_set_owner(sem);
@@ -1385,12 +1388,10 @@ static inline int __down_write_killable(struct rw_semaphore *sem)

static inline int __down_write_trylock(struct rw_semaphore *sem)
{
- long tmp;
+ long tmp = RWSEM_UNLOCKED_VALUE;

- lockevent_inc(rwsem_wtrylock);
- tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
- RWSEM_WRITER_LOCKED);
- if (tmp == RWSEM_UNLOCKED_VALUE) {
+ if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
+ RWSEM_WRITER_LOCKED)) {
rwsem_set_owner(sem);
return true;
}
@@ -1407,12 +1408,9 @@ inline void __up_read(struct rw_semaphore *sem)
DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
rwsem_clear_reader_owned(sem);
tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
- if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS))
- == RWSEM_FLAG_WAITERS)) {
- /* Clear RWSEM_WR_UNSPINNABLE bit if set */
- if (!is_rwsem_spinnable(sem, true))
- atomic_long_andnot(RWSEM_WR_NONSPINNABLE,
- (atomic_long_t *)&sem->owner);
+ if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
+ RWSEM_FLAG_WAITERS)) {
+ clear_wr_nonspinnable(sem);
rwsem_wake(sem, tmp);
}
}