[PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation
From: Waiman Long
Date: Tue Jan 28 2014 - 13:20:20 EST
This patch introduces a new queue spinlock implementation that can
serve as an alternative to the default ticket spinlock. Compared with
the ticket spinlock, this queue spinlock should be almost as fair as
the ticket spinlock. It has about the same speed in single-thread and
it can be much faster in high contention situations. Only in light to
moderate contention where the average queue depth is around 1-2 will
this queue spinlock be potentially a bit slower due to the higher
slowpath overhead.
This queue spinlock is especially suit to NUMA machines with a large
number of cores as the chance of spinlock contention is much higher
in those machines. The cost of contention is also higher because of
slower inter-node memory traffic.
The idea behind this spinlock implementation is the fact that spinlocks
are acquired with preemption disabled. In other words, the process
will not be migrated to another CPU while it is trying to get a
spinlock. Ignoring interrupt handling, a CPU can only be contending
in one spinlock at any one time. Of course, interrupt handler can try
to acquire one spinlock while the interrupted user process is in the
process of getting another spinlock. By allocating a set of per-cpu
queue nodes and used them to form a waiting queue, we can encode the
queue node address into a much smaller 16-bit size. Together with
the 1-byte lock bit, this queue spinlock implementation will only
need 4 bytes to hold all the information that it needs.
The current queue node address encoding of the 4-byte word is as
follows:
Bits 0-7 : the locked byte
Bits 8-9 : queue node index in the per-cpu array (4 entries)
Bits 10-31: cpu number + 1 (max cpus = 4M -1)
In the extremely unlikely case that all the queue node entries are
used up, the current code will fall back to busy spinning without
waiting in a queue with warning message.
For single-thread performance (no contention), a 256K lock/unlock
loop was run on a 2.4Ghz Westmere x86-64 CPU. The following table
shows the average time (in ns) for a single lock/unlock sequence
(including the looping and timing overhead):
Lock Type Time (ns)
--------- ---------
Ticket spinlock 14.1
Queue spinlock (Normal) 8.8*
So the queue spinlock is much faster than the ticket spinlock, even
though the overhead of locking and unlocking should be pretty small
when there is no contention. (* That is with the barrier.h patch that
extends __native_word() to char and short.)
The AIM7 benchmark was run on a 8-socket 80-core DL980 with Westmere
x86-64 CPUs with XFS filesystem on a ramdisk and HT off to evaluate
the performance impact of this patch on a 3.13 kernel.
+------------+----------+-----------------+---------+
| Kernel | 3.13 JPM | 3.13 with | %Change |
| | | qspinlock patch | |
+------------+----------+-----------------+---------+
| 10-100 users |
+------------+----------+-----------------+---------+
|custom | 357459 | 363109 | +1.58% |
|dbase | 496847 | 498801 | +0.39% |
|disk | 2925312 | 2771387 | -5.26% |
|five_sec | 166612 | 169215 | +1.56% |
|fserver | 382129 | 383279 | +0.30% |
|high_systime| 16356 | 16380 | +0.15% |
|short | 4521978 | 4257363 | -5.85% |
+------------+----------+-----------------+---------+
| 200-1000 users |
+------------+----------+-----------------+---------+
|custom | 449070 | 447711 | -0.30% |
|dbase | 845029 | 853362 | +0.99% |
|disk | 2725249 | 4892907 | +79.54% |
|five_sec | 169410 | 170638 | +0.72% |
|fserver | 489662 | 491828 | +0.44% |
|high_systime| 142823 | 143790 | +0.68% |
|short | 7435288 | 9016171 | +21.26% |
+------------+----------+-----------------+---------+
| 1100-2000 users |
+------------+----------+-----------------+---------+
|custom | 432470 | 432570 | +0.02% |
|dbase | 889289 | 890026 | +0.08% |
|disk | 2565138 | 5008732 | +95.26% |
|five_sec | 169141 | 170034 | +0.53% |
|fserver | 498569 | 500701 | +0.43% |
|high_systime| 229913 | 245866 | +6.94% |
|short | 8496794 | 8281918 | -2.53% |
+------------+----------+-----------------+---------+
The workload with the most gain was the disk workload. Without the
patch, the perf profile at 1500 users looked like:
26.19% reaim [kernel.kallsyms] [k] _raw_spin_lock
|--47.28%-- evict
|--46.87%-- inode_sb_list_add
|--1.24%-- xlog_cil_insert_items
|--0.68%-- __remove_inode_hash
|--0.67%-- inode_wait_for_writeback
--3.26%-- [...]
22.96% swapper [kernel.kallsyms] [k] cpu_idle_loop
5.56% reaim [kernel.kallsyms] [k] mutex_spin_on_owner
4.87% reaim [kernel.kallsyms] [k] update_cfs_rq_blocked_load
2.04% reaim [kernel.kallsyms] [k] mspin_lock
1.30% reaim [kernel.kallsyms] [k] memcpy
1.08% reaim [unknown] [.] 0x0000003c52009447
There was pretty high spinlock contention on the inode_sb_list_lock
and maybe the inode's i_lock.
With the patch, the perf profile at 1500 users became:
26.82% swapper [kernel.kallsyms] [k] cpu_idle_loop
4.66% reaim [kernel.kallsyms] [k] mutex_spin_on_owner
3.97% reaim [kernel.kallsyms] [k] update_cfs_rq_blocked_load
2.40% reaim [kernel.kallsyms] [k] queue_spin_lock_slowpath
|--88.31%-- _raw_spin_lock
| |--36.02%-- inode_sb_list_add
| |--35.09%-- evict
| |--16.89%-- xlog_cil_insert_items
| |--6.30%-- try_to_wake_up
| |--2.20%-- _xfs_buf_find
| |--0.75%-- __remove_inode_hash
| |--0.72%-- __mutex_lock_slowpath
| |--0.53%-- load_balance
|--6.02%-- _raw_spin_lock_irqsave
| |--74.75%-- down_trylock
| |--9.69%-- rcu_check_quiescent_state
| |--7.47%-- down
| |--3.57%-- up
| |--1.67%-- rwsem_wake
| |--1.00%-- remove_wait_queue
| |--0.56%-- pagevec_lru_move_fn
|--5.39%-- _raw_spin_lock_irq
| |--82.05%-- rwsem_down_read_failed
| |--10.48%-- rwsem_down_write_failed
| |--4.24%-- __down
| |--2.74%-- __schedule
--0.28%-- [...]
2.20% reaim [kernel.kallsyms] [k] memcpy
1.84% reaim [unknown] [.] 0x000000000041517b
1.77% reaim [kernel.kallsyms] [k] _raw_spin_lock
|--21.08%-- xlog_cil_insert_items
|--10.14%-- xfs_icsb_modify_counters
|--7.20%-- xfs_iget_cache_hit
|--6.56%-- inode_sb_list_add
|--5.49%-- _xfs_buf_find
|--5.25%-- evict
|--5.03%-- __remove_inode_hash
|--4.64%-- __mutex_lock_slowpath
|--3.78%-- selinux_inode_free_security
|--2.95%-- xfs_inode_is_filestream
|--2.35%-- try_to_wake_up
|--2.07%-- xfs_inode_set_reclaim_tag
|--1.52%-- list_lru_add
|--1.16%-- xfs_inode_clear_eofblocks_tag
:
1.30% reaim [kernel.kallsyms] [k] effective_load
1.27% reaim [kernel.kallsyms] [k] mspin_lock
1.10% reaim [kernel.kallsyms] [k] security_compute_sid
On the ext4 filesystem, the disk workload improved from 416281 JPM
to 899101 JPM (+116%) with the patch. In this case, the contended
spinlock is the mb_cache_spinlock.
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
include/asm-generic/qspinlock.h | 156 +++++++++++++++
kernel/Kconfig.locks | 7 +
kernel/locking/Makefile | 1 +
kernel/locking/qspinlock.c | 398 +++++++++++++++++++++++++++++++++++++++
4 files changed, 562 insertions(+), 0 deletions(-)
create mode 100644 include/asm-generic/qspinlock.h
create mode 100644 kernel/locking/qspinlock.c
diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
new file mode 100644
index 0000000..536e568
--- /dev/null
+++ b/include/asm-generic/qspinlock.h
@@ -0,0 +1,156 @@
+/*
+ * Queue spinlock
+ *
+ * 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.long@xxxxxx>
+ */
+#ifndef __ASM_GENERIC_QSPINLOCK_H
+#define __ASM_GENERIC_QSPINLOCK_H
+
+#include <linux/types.h>
+#include <linux/atomic.h>
+#include <asm/cmpxchg.h>
+#include <asm/barrier.h>
+#include <asm/processor.h>
+#include <asm/byteorder.h>
+
+#if !defined(__LITTLE_ENDIAN) && !defined(__BIG_ENDIAN)
+#error "Missing either LITTLE_ENDIAN or BIG_ENDIAN definition."
+#endif
+
+/*
+ * The queue spinlock data structure - a 32-bit word
+ * Bits 0-7 : Set if locked
+ * Bits 8-31: Queue code
+ */
+typedef struct qspinlock {
+ union {
+ u32 qlcode; /* Queue spinlock code */
+ atomic_t qlcode_a; /* Atomic_t version */
+ struct {
+#ifdef __LITTLE_ENDIAN
+ u8 locked; /* 8-bit lock word */
+#else
+ u16 qcode16; /* MS wait queue code */
+ u8 qcode8; /* LS wait queue code */
+ u8 locked; /* 8-bit lock word */
+#endif
+ };
+ };
+} arch_spinlock_t;
+
+#define _QSPINLOCK_LOCKED 1
+#define _QCODE_OFFSET 8
+#define _QCODE(lock) (ACCESS_ONCE((lock)->qlcode) >> _QCODE_OFFSET)
+
+/*
+ * External function declarations
+ */
+extern void queue_spin_lock_slowpath(struct qspinlock *lock);
+
+/**
+ * queue_spin_is_locked - is the spinlock locked?
+ * @lock: Pointer to queue spinlock structure
+ * Return: 1 if it is locked, 0 otherwise
+ */
+static __always_inline int queue_spin_is_locked(struct qspinlock *lock)
+{
+ return ACCESS_ONCE(lock->locked);
+}
+
+/**
+ * queue_spin_value_unlocked - is the spinlock structure unlocked?
+ * @lock: queue spinlock structure
+ * Return: 1 if it is unlocked, 0 otherwise
+ */
+static __always_inline int queue_spin_value_unlocked(struct qspinlock lock)
+{
+ return !lock.locked;
+}
+
+/**
+ * queue_spin_is_contended - check if the lock is contended
+ * @lock : Pointer to queue spinlock structure
+ * Return: 1 if lock contended, 0 otherwise
+ */
+static __always_inline int queue_spin_is_contended(struct qspinlock *lock)
+{
+ return _QCODE(lock);
+}
+/**
+ * queue_spin_trylock - try to acquire the queue spinlock
+ * @lock : Pointer to queue spinlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+static __always_inline int queue_spin_trylock(struct qspinlock *lock)
+{
+ if (!ACCESS_ONCE(lock->qlcode) &&
+ (cmpxchg(&lock->qlcode, 0, _QSPINLOCK_LOCKED) == 0))
+ return 1;
+ return 0;
+}
+
+/**
+ * queue_spin_lock - acquire a queue spinlock
+ * @lock: Pointer to queue spinlock structure
+ */
+static __always_inline void queue_spin_lock(struct qspinlock *lock)
+{
+ /*
+ * To reduce memory access to only once for the cold cache case,
+ * a direct cmpxchg() is performed in the fastpath to optimize the
+ * uncontended case. The contended performance, however, may suffer
+ * a bit because of that.
+ */
+ if (likely(cmpxchg(&lock->qlcode, 0, _QSPINLOCK_LOCKED) == 0))
+ return;
+ queue_spin_lock_slowpath(lock);
+}
+
+/**
+ * queue_spin_unlock - release a queue spinlock
+ * @lock : Pointer to queue spinlock structure
+ */
+static __always_inline void queue_spin_unlock(struct qspinlock *lock)
+{
+ /*
+ * If the writer field is atomic, it can be cleared directly.
+ * Otherwise, an atomic subtraction will be used to clear it.
+ */
+ if (__native_word(lock->locked))
+ smp_store_release(&lock->locked, 0);
+ else {
+ smp_mb__before_atomic_dec();
+ atomic_sub(_QSPINLOCK_LOCKED, &lock->qlcode_a);
+ }
+}
+
+/*
+ * Initializier
+ */
+#define __ARCH_SPIN_LOCK_UNLOCKED { { 0 } }
+
+/*
+ * Remapping spinlock architecture specific functions to the corresponding
+ * queue spinlock functions.
+ */
+#define arch_spin_is_locked(l) queue_spin_is_locked(l)
+#define arch_spin_is_contended(l) queue_spin_is_contended(l)
+#define arch_spin_value_unlocked(l) queue_spin_value_unlocked(l)
+#define arch_spin_lock(l) queue_spin_lock(l)
+#define arch_spin_trylock(l) queue_spin_trylock(l)
+#define arch_spin_unlock(l) queue_spin_unlock(l)
+#define arch_spin_lock_flags(l, f) queue_spin_lock(l)
+
+#endif /* __ASM_GENERIC_QSPINLOCK_H */
diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
index d2b32ac..f185584 100644
--- 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_SPINLOCK
+ bool
+
+config QUEUE_SPINLOCK
+ def_bool y if ARCH_USE_QUEUE_SPINLOCK
+ depends on SMP && !PARAVIRT_SPINLOCKS
diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index baab8e5..1a17380 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -23,3 +23,4 @@ obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock_debug.o
obj-$(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_SPINLOCK) += qspinlock.o
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
new file mode 100644
index 0000000..da4b6cc
--- /dev/null
+++ b/kernel/locking/qspinlock.c
@@ -0,0 +1,398 @@
+/*
+ * Queue spinlock
+ *
+ * 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.long@xxxxxx>
+ */
+#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 <asm-generic/qspinlock.h>
+
+/*
+ * The basic principle of a queue-based spinlock can best be understood
+ * by studying a classic queue-based spinlock implementation called the
+ * MCS lock. The paper below provides a good description for this kind
+ * of lock.
+ *
+ * http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf
+ *
+ * This queue spinlock implementation is based on the MCS lock with twists
+ * to make it fit the following constraints:
+ * 1. A max spinlock size of 4 bytes
+ * 2. Good fastpath performance
+ * 3. No change in the locking APIs
+ *
+ * The queue spinlock fastpath is as simple as it can get, all the heavy
+ * lifting is done in the lock slowpath. The main idea behind this queue
+ * spinlock implementation is to keep the spinlock size at 4 bytes while
+ * at the same time implement a queue structure to queue up the waiting
+ * lock spinners.
+ *
+ * Since preemption is disabled before getting the lock, a given CPU will
+ * only need to use one queue node structure in a non-interrupt context.
+ * A percpu queue node structure will be allocated for this purpose and the
+ * cpu number will be put into the queue spinlock structure to indicate the
+ * tail of the queue.
+ *
+ * To handle spinlock acquisition at interrupt context (softirq or hardirq),
+ * the queue node structure is actually an array for supporting nested spin
+ * locking operations in interrupt handlers. If all the entries in the
+ * array are used up, a warning message will be printed (as that shouldn't
+ * happen in normal circumstances) and the lock spinner will fall back to
+ * busy spinning instead of waiting in a queue.
+ */
+
+/*
+#ifndef CONFIG_DEBUG_SPINLOCK
+#define CONFIG_DEBUG_SPINLOCK 1
+#endif
+ */
+
+/*
+ * The queue node structure
+ *
+ * The used flag is used for synchronization between processs and interrupt
+ * contexts of the same CPU. So it should be set first at initialization and
+ * cleared last in the cleanup code.
+ */
+struct qnode {
+ u32 used; /* Used flag */
+ u32 wait; /* Waiting flag */
+#ifdef CONFIG_DEBUG_SPINLOCK
+ u32 cpu_nr; /* CPU number */
+ void *lock; /* Lock address */
+#endif
+ struct qnode *next; /* Next queue node addr */
+};
+
+/*
+ * The 24-bit queue node code is divided into the following 2 fields:
+ * Bits 0-1 : queue node index (4 nodes)
+ * Bits 2-23: CPU number + 1 (4M - 1 CPUs)
+ *
+ * A queue node code of 0 indicates that no one is waiting for the lock.
+ * As the value 0 cannot be used as a valid CPU number. We need to add
+ * 1 to it before putting it into the queue code.
+ */
+#define MAX_QNODES 4
+#define GET_QN_IDX(code) (((code) >> 8) & 3)
+#define GET_CPU_NR(code) (((code) >> 10) - 1)
+#define SET_QCODE(cpu, idx) ((((cpu) + 1) << 10) | ((idx) << 8) |\
+ _QSPINLOCK_LOCKED)
+
+/*
+ * Per-CPU queue node structures
+ */
+static DEFINE_PER_CPU(struct qnode [MAX_QNODES], qnodes) ____cacheline_aligned
+ = { { 0 } };
+
+/**
+ * xlate_qcode - translate the queue code into the queue node address
+ * @qcode: Queue code to be translated
+ * Return: The corresponding queue node address
+ */
+static inline struct qnode *xlate_qcode(u32 qcode)
+{
+ u32 cpu_nr = GET_CPU_NR(qcode);
+ u8 qn_idx = GET_QN_IDX(qcode);
+
+ return per_cpu_ptr(&qnodes[qn_idx], cpu_nr);
+}
+
+/*
+ * Debugging macros & functions
+ */
+#ifdef CONFIG_DEBUG_SPINLOCK
+#define ASSERT(cond) BUG_ON(!(cond))
+#define SET_NODE(var, val) do { node->(var) = (val); } while (0)
+
+/**
+ * assert_nextnode - assert that the next node is valid
+ * @next: Pointer to the next node
+ * @lock: Pointer to queue spinlock structure
+ * @cpu_nr: CPU number
+ */
+static noinline void
+assert_nextnode(struct qnode *next, struct qspinlock *lock, u32 cpu_nr)
+{
+ ASSERT(next->cpu_nr != cpu_nr);
+ ASSERT(next->lock == (void *)lock);
+ ASSERT(next->wait);
+ ASSERT(next->used);
+}
+
+
+/**
+ * assert_prevnode - assert the previous node is valid
+ * @prev: Pointer to the previous node
+ * @lock: Pointer to queue spinlock structure
+ * @cpu_nr: CPU number
+ */
+static noinline void
+assert_prevnode(struct qnode *prev, struct qspinlock *lock, u32 cpu_nr)
+{
+ ASSERT(prev->cpu_nr != cpu_nr);
+ ASSERT(prev->lock == (void *)lock);
+ ASSERT(prev->used);
+ ASSERT(prev->next == NULL);
+}
+
+#else
+#define ASSERT(cond)
+#define SET_NODE(var, val)
+#define assert_nextnode(next, lock, cpu_nr)
+#define assert_prevnode(prev, lock, cpu_nr)
+#endif
+
+/**
+ * unfair_trylock - try to acquire the lock ignoring the qcode
+ * @lock: Pointer to queue spinlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+static __always_inline int unfair_trylock(struct qspinlock *lock)
+{
+ u32 qlcode = ACCESS_ONCE(lock->qlcode);
+
+ if (qlcode & _QSPINLOCK_LOCKED)
+ return 0;
+ if (__native_word(lock->locked)) {
+ if (cmpxchg(&lock->locked, 0, _QSPINLOCK_LOCKED) == 0)
+ return 1;
+ } else if (cmpxchg(&lock->qlcode, qlcode, qlcode|_QSPINLOCK_LOCKED)
+ == qlcode)
+ return 1;
+ return 0;
+}
+
+/**
+ * init_node - initialize the queue node
+ * @node: Pointer to queue node structure
+ * @lock: Pointer to queue spinlock structure
+ * @cpu_nr: CPU number
+ *
+ * The used flag must be set before other fields. Since the used flag will
+ * always be checked by the same CPU, even though it may be in a different
+ * interrupt context, no explicit barrier instruction other than a compiler
+ * barrier will be needed.
+ */
+static inline void
+init_node(struct qnode *node, struct qspinlock *lock, u32 cpu_nr)
+{
+ ASSERT(!node->used);
+ node->used = true;
+ barrier();
+
+ ASSERT(!node->lock);
+ ASSERT(!node->next);
+ ASSERT(!node->wait);
+ node->wait = true;
+ node->next = NULL;
+ SET_NODE(cpu_nr, cpu_nr);
+ SET_NODE(lock, (void *)lock);
+}
+
+/**
+ * cleanup_node - Clean up the queue node
+ * @node: Pointer to queue node structure
+ * @cpu_nr: CPU number
+ *
+ * The used flag must be the last one to be cleared.
+ */
+static inline void cleanup_node(struct qnode *node, u16 cpu_nr)
+{
+ node->next = NULL;
+ node->wait = false;
+ SET_NODE(lock, NULL);
+ ASSERT(cpu_nr == smp_processor_id());
+ barrier();
+ node->used = false;
+}
+
+/**
+ * queue_spin_lock_slowpath - acquire the queue spinlock
+ * @lock: Pointer to queue spinlock structure
+ */
+void queue_spin_lock_slowpath(struct qspinlock *lock)
+{
+ unsigned int cpu_nr, qn_idx;
+ struct qnode *node, *next = NULL;
+ u32 prev_qcode, my_qcode;
+
+ /*
+ * Get the queue node
+ */
+ cpu_nr = smp_processor_id();
+ node = this_cpu_ptr(&qnodes[0]);
+ qn_idx = 0;
+
+ if (unlikely(node->used)) {
+ /*
+ * This node has been used, try to find an empty queue
+ * node entry.
+ */
+ for (qn_idx = 1; qn_idx < MAX_QNODES; qn_idx++)
+ if (!node[qn_idx].used)
+ break;
+ if (unlikely(qn_idx == MAX_QNODES)) {
+ /*
+ * This shouldn't happen, print a warning message
+ * & busy spinning on the lock.
+ */
+ printk_sched(
+ "qspinlock: queue node table exhausted at cpu %d!\n",
+ cpu_nr);
+ while (!unfair_trylock(lock))
+ arch_mutex_cpu_relax();
+ return;
+ }
+ /* Adjust node pointer */
+ node += qn_idx;
+ }
+
+ /*
+ * Set up the new cpu code to be exchanged
+ */
+ my_qcode = SET_QCODE(cpu_nr, qn_idx);
+
+ /*
+ * The lock may be available at this point, try again before waiting
+ * in a queue.
+ */
+ if (queue_spin_trylock(lock))
+ return;
+
+ /*
+ * Initialize the queue node
+ */
+ init_node(node, lock, cpu_nr);
+
+ /*
+ * Exchange current copy of the queue node code
+ */
+ prev_qcode = xchg(&lock->qlcode, my_qcode);
+ /*
+ * It is possible that we may accidentally steal the lock. If this is
+ * the case, we need to either release it if not the head of the queue
+ * or get the lock and be done with it.
+ */
+ if (unlikely(!(prev_qcode & _QSPINLOCK_LOCKED))) {
+ if (prev_qcode == 0) {
+ /*
+ * Got the lock since it is at the head of the queue
+ * Now try to atomically clear the queue code.
+ */
+ if (cmpxchg(&lock->qlcode, my_qcode, _QSPINLOCK_LOCKED)
+ == my_qcode)
+ goto release_node;
+ /*
+ * The cmpxchg fails only if one or more processes
+ * are added to the queue. In this case, we need to
+ * notify the next one to be the head of the queue.
+ */
+ goto notify_next;
+ }
+ /*
+ * Accidentally steal the lock, release the lock and
+ * let the queue head get it.
+ */
+ queue_spin_unlock(lock);
+ } else
+ prev_qcode &= ~_QSPINLOCK_LOCKED; /* Clear the lock bit */
+ my_qcode &= ~_QSPINLOCK_LOCKED;
+
+ if (prev_qcode) {
+ /*
+ * Not at the queue head, get the address of the previous node
+ * and set up the "next" fields of the that node.
+ */
+ struct qnode *prev = xlate_qcode(prev_qcode);
+
+ assert_prevnode(prev, lock, cpu_nr);
+ ACCESS_ONCE(prev->next) = node;
+ /*
+ * Wait until the waiting flag is off
+ */
+ while (smp_load_acquire(&node->wait))
+ arch_mutex_cpu_relax();
+ }
+
+ /*
+ * At the head of the wait queue now
+ */
+ while (true) {
+ u32 qcode;
+
+ if (unlikely(!next)) {
+ /*
+ * Try to get the next node address & clean up
+ * current node data structure now if the next node
+ * address had been set.
+ */
+ next = ACCESS_ONCE(node->next);
+ if (next) {
+ assert_nextnode(next, lock, cpu_nr);
+ cleanup_node(node, cpu_nr);
+ node = NULL;
+ }
+ }
+ qcode = ACCESS_ONCE(lock->qlcode);
+ if (qcode & _QSPINLOCK_LOCKED)
+ ; /* Lock not available yet */
+ else if (qcode != my_qcode) {
+ /*
+ * Just get the lock with other spinners waiting
+ * in the queue.
+ */
+ if (unfair_trylock(lock))
+ /* May still need to notify the next node */
+ goto notify_next;
+ } else {
+ /*
+ * Get the lock & clear the queue code simultaneously
+ */
+ if (cmpxchg(&lock->qlcode, my_qcode,
+ _QSPINLOCK_LOCKED) == my_qcode)
+ /* No need to notify next one */
+ goto release_node;
+ }
+ arch_mutex_cpu_relax();
+ }
+
+notify_next:
+ /*
+ * If the next pointer is not set, we need to wait and notify the
+ * next one in line to do busy spinning.
+ */
+ if (unlikely(!next)) {
+ /*
+ * Wait until the next one in queue set up the next field
+ */
+ while (!(next = ACCESS_ONCE(node->next)))
+ arch_mutex_cpu_relax();
+ assert_nextnode(next, lock, cpu_nr);
+ }
+ /*
+ * The next one in queue is now at the head
+ */
+ smp_store_release(&next->wait, false);
+
+release_node:
+ if (node)
+ cleanup_node(node, cpu_nr);
+}
+EXPORT_SYMBOL(queue_spin_lock_slowpath);
--
1.7.1
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