So I took out that ugly union and rewrote the code to be mostly
atomic_*(), gcc generates acceptable code and its smaller too.
824 0 0 824 338 defconfig-build/kernel/locking/qrwlock.o
776 0 0 776 308 defconfig-build/kernel/locking/qrwlock.o
I don't think I wrecked it, but I've not actually tried it yet.
For now it uses the atomic_sub() unlock path, but by using a #ifndef we
can have arch overrides for that.
Eg, x86 could have something like:
#if !defined(CONFIG_X86_OOSTORE) && !defined(CONFIG_X86_PPRO_FENCE)
#define queue_write_unlock queue_write_unlock
static inline void queue_write_unlock(struct qrwlock *lock)
{
barrier();
ACCESS_ONCE(*(u8 *)&lock->cnts) = 0;
}
#endif
---
--- /dev/null
+++ b/include/asm-generic/qrwlock.h
@@ -0,0 +1,40 @@
+/*
+ * Queue read/write lock
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
+ *
+ * Authors: Waiman Long <waiman.l...@hp.com>
+ */
+#ifndef __ASM_GENERIC_QRWLOCK_H
+#define __ASM_GENERIC_QRWLOCK_H
+
+
+/*
+ * Initializier
+ */
+#define __ARCH_RW_LOCK_UNLOCKED { .cnts = { .rwc = 0 }, .waitq = NULL }
+
+/*
+ * Remapping rwlock architecture specific functions to the corresponding
+ * queue rwlock functions.
+ */
+#define arch_read_can_lock(l) queue_read_can_lock(l)
+#define arch_write_can_lock(l) queue_write_can_lock(l)
+#define arch_read_lock(l) queue_read_lock(l)
+#define arch_write_lock(l) queue_write_lock(l)
+#define arch_read_trylock(l) queue_read_trylock(l)
+#define arch_write_trylock(l) queue_write_trylock(l)
+#define arch_read_unlock(l) queue_read_unlock(l)
+#define arch_write_unlock(l) queue_write_unlock(l)
+
+#endif /* __ASM_GENERIC_QRWLOCK_H */
--- a/kernel/Kconfig.locks
+++ b/kernel/Kconfig.locks
@@ -223,3 +223,10 @@ endif
config MUTEX_SPIN_ON_OWNER
def_bool y
depends on SMP && !DEBUG_MUTEXES
+
+config ARCH_USE_QUEUE_RWLOCK
+ bool
+
+config QUEUE_RWLOCK
+ def_bool y if ARCH_USE_QUEUE_RWLOCK
+ depends on SMP
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -23,3 +23,4 @@ obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock
obj-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
obj-$(CONFIG_PERCPU_RWSEM) += percpu-rwsem.o
+obj-$(CONFIG_QUEUE_RWLOCK) += qrwlock.o
--- /dev/null
+++ b/kernel/locking/qrwlock.c
@@ -0,0 +1,310 @@
+/*
+ * Queue read/write lock
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
+ *
+ * Authors: Waiman Long <waiman.l...@hp.com>
+ */
+#include <linux/smp.h>
+#include <linux/bug.h>
+#include <linux/cpumask.h>
+#include <linux/percpu.h>
+#include <linux/hardirq.h>
+#include <linux/mutex.h>
+#include <linux/mcs_spinlock.h>
+// #include <asm-generic/qrwlock.h>
+
+
+// XXX stolen from header to get a single translation unit to compare
+
+#include <linux/types.h>
+#include <linux/atomic.h>
+#include <asm/barrier.h>
+#include <asm/processor.h>
+
+/*
+ * The queue read/write lock data structure
+ *
+ * The layout of the structure is endian-sensitive to make sure that adding
+ * _QR_BIAS to the rw field to increment the reader count won't disturb
+ * the writer field. The least significant 8 bits is the writer field
+ * whereas the remaining 24 bits is the reader count.
+ */
+struct mcs_spinlock;
+
+struct qrwlock {
+ atomic_t cnts; /* Reader/writer atomic */
+ struct mcs_spinlock *waitq; /* Tail of waiting queue */
+};
+
+/*
+ * Writer states & reader shift and bias
+ */
+#define _QW_WAITING 1 /* A writer is waiting */
+#define _QW_LOCKED 0xff /* A writer holds the lock */
+#define _QW_WMASK 0xff /* Writer mask */
+#define _QR_SHIFT 8 /* Reader count shift */
+#define _QR_BIAS (1U << _QR_SHIFT)
+
+/*
+ * External function declarations
+ */
+extern void queue_read_lock_slowpath(struct qrwlock *lock);
+extern void queue_write_lock_slowpath(struct qrwlock *lock);
+
+/**
+ * queue_read_can_lock- would read_trylock() succeed?
+ * @lock: Pointer to queue rwlock structure
+ */
+inline int queue_read_can_lock(struct qrwlock *lock)
+{
+ return !(atomic_read(&lock->cnts) & _QW_WMASK);
+}
+
+/**
+ * queue_write_can_lock- would write_trylock() succeed?
+ * @lock: Pointer to queue rwlock structure
+ */
+inline int queue_write_can_lock(struct qrwlock *lock)
+{
+ return !atomic_read(&lock->cnts);
+}
+
+/**
+ * queue_read_trylock - try to acquire read lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+inline int queue_read_trylock(struct qrwlock *lock)
+{
+ u32 cnts;
+
+ cnts = atomic_read(&lock->cnts);
+ if (likely(!(cnts & _QW_WMASK))) {
+ cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
+ if (likely(!(cnts & _QW_WMASK)))
+ return 1;
+ atomic_sub(_QR_BIAS, &lock->cnts);
+ }
+ return 0;
+}
+
+/**
+ * queue_write_trylock - try to acquire write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+inline int queue_write_trylock(struct qrwlock *lock)
+{
+ u32 old, new;
+
+ old = atomic_read(&lock->cnts);
+ if (unlikely(old))
+ return 0;
+
+ new = old | _QW_LOCKED;
+ return likely(atomic_cmpxchg(&lock->cnts, old, new) == old);
+}
+/**
+ * queue_read_lock - acquire read lock of a queue rwlock
+ * @lock: Pointer to queue rwlock structure
+ */
+inline void queue_read_lock(struct qrwlock *lock)
+{
+ u32 cnts;
+
+ cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
+ if (likely(!(cnts & _QW_WMASK)))
+ return;
+
+ /* The slowpath will decrement the reader count, if necessary. */
+ queue_read_lock_slowpath(lock);
+}
+
+/**
+ * queue_write_lock - acquire write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+inline void queue_write_lock(struct qrwlock *lock)
+{
+ /* Optimize for the unfair lock case where the fair flag is 0. */
+ if (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)
+ return;
+
+ queue_write_lock_slowpath(lock);
+}
+
+/**
+ * queue_read_unlock - release read lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+inline void queue_read_unlock(struct qrwlock *lock)
+{
+ /*
+ * Atomically decrement the reader count
+ */
+ smp_mb__before_atomic_dec();
+ atomic_sub(_QR_BIAS, &lock->cnts);
+}
+
+#ifndef queue_write_unlock
+/**
+ * queue_write_unlock - release write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+inline void queue_write_unlock(struct qrwlock *lock)
+{
+ /*
+ * If the writer field is atomic, it can be cleared directly.
+ * Otherwise, an atomic subtraction will be used to clear it.
+ */
+ smp_mb__before_atomic_dec();
+ atomic_sub(_QW_LOCKED, &lock->cnts);
+}
+#endif
+
+// XXX
+
+/*
+ * Compared with regular rwlock, the queue rwlock has has the following
+ * advantages:
+ * 1. Even though there is a slight chance of stealing the lock if come at
+ * the right moment, the granting of the lock is mostly in FIFO order.
+ * 2. It is usually faster in high contention situation.
+ *
+ * The only downside is that the lock is 4 bytes larger in 32-bit systems
+ * and 12 bytes larger in 64-bit systems.
+ *
+ * There are two queues for writers. The writer field of the lock is a
+ * one-slot wait queue. The writers that follow will have to wait in the
+ * combined reader/writer queue (waitq).
+ *
+ * Compared with x86 ticket spinlock, the queue rwlock is faster in high
+ * contention situation. The writer lock is also faster in single thread
+ * operations. Therefore, queue rwlock can be considered as a replacement
+ * for those spinlocks that are highly contended as long as an increase
+ * in lock size is not an issue.
+ */
+
+/**
+ * rspin_until_writer_unlock - inc reader count & spin until writer is gone
+ * @lock : Pointer to queue rwlock structure
+ * @writer: Current queue rwlock writer status byte
+ *
+ * In interrupt context or at the head of the queue, the reader will just
+ * increment the reader count & wait until the writer releases the lock.
+ */
+static __always_inline void
+rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
+{
+ while ((cnts & _QW_WMASK) == _QW_LOCKED) {
+ arch_mutex_cpu_relax();
+ cnts = smp_load_acquire((u32 *)&lock->cnts);
+ }
+}
+
+/**
+ * queue_read_lock_slowpath - acquire read lock of a queue rwlock
+ * @lock: Pointer to queue rwlock structure
+ */
+void queue_read_lock_slowpath(struct qrwlock *lock)
+{
+ struct mcs_spinlock node;
+ u32 cnts;
+
+ /*
+ * Readers come here when they cannot get the lock without waiting
+ */
+ if (unlikely(irq_count())) {
+ /*
+ * Readers in interrupt context will spin until the lock is
+ * available without waiting in the queue.
+ */
+ cnts = smp_load_acquire((u32 *)&lock->cnts);
+ rspin_until_writer_unlock(lock, cnts);
+ return;
+ }
+ atomic_sub(_QR_BIAS, &lock->cnts);
+
+ /*
+ * Put the reader into the wait queue
+ */
+ mcs_spin_lock(&lock->waitq, &node);
+
+ /*
+ * At the head of the wait queue now, wait until the writer state
+ * goes to 0 and then try to increment the reader count and get
+ * the lock. It is possible that an incoming writer may steal the
+ * lock in the interim, so it is necessary to check the writer byte
+ * to make sure that the write lock isn't taken.
+ */
+ while (atomic_read(&lock->cnts) & _QW_WMASK)
+ arch_mutex_cpu_relax();
+
+ cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
+ rspin_until_writer_unlock(lock, cnts);
+
+ /*
+ * Signal the next one in queue to become queue head
+ */
+ mcs_spin_unlock(&lock->waitq, &node);
+}
+EXPORT_SYMBOL(queue_read_lock_slowpath);
+
+/**
+ * queue_write_lock_slowpath - acquire write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+void queue_write_lock_slowpath(struct qrwlock *lock)
+{
+ struct mcs_spinlock node;
+ u32 cnts, old;
+
+ /* Put the writer into the wait queue */
+ mcs_spin_lock(&lock->waitq, &node);
+
+ /* Try to acquire the lock directly if no reader is present */
+ if (!atomic_read(&lock->cnts) &&
+ (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0))
+ goto unlock;
+
+ /*
+ * Set the waiting flag to notify readers that a writer is pending,
+ * or wait for a previous writer to go away.
+ */
+ cnts = atomic_read(&lock->cnts);
+ for (;;) {
+ if (!(cnts & _QW_WMASK) &&
+ ((old = atomic_cmpxchg(&lock->cnts,
+ cnts, cnts | _QW_WAITING)) == cnts))
+ break;
+
+ cnts = old;
+ arch_mutex_cpu_relax();
+ }
+
+ /* When no more readers, set the locked flag */
+ cnts = atomic_read(&lock->cnts);
+ for (;;) {
+ if ((cnts == _QW_WAITING) &&
+ ((old = atomic_cmpxchg(&lock->cnts,
+ _QW_WAITING, _QW_LOCKED)) == _QW_WAITING))
+ break;
+
+ cnts = old;
+ arch_mutex_cpu_relax();
+ }
+unlock:
+ mcs_spin_unlock(&lock->waitq, &node);
+}
+EXPORT_SYMBOL(queue_write_lock_slowpath);
--
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