Generic Mutex Subsystem
started by Ingo Molnar <mingo@redhat.com>
+updated by Davidlohr Bueso <davidlohr@hp.com>
- "Why on earth do we need a new mutex subsystem, and what's wrong
- with semaphores?"
+What are mutexes?
+-----------------
-firstly, there's nothing wrong with semaphores. But if the simpler
-mutex semantics are sufficient for your code, then there are a couple
-of advantages of mutexes:
+In the Linux kernel, mutexes refer to a particular locking primitive
+that enforces serialization on shared memory systems, and not only to
+the generic term referring to 'mutual exclusion' found in academia
+or similar theoretical text books. Mutexes are sleeping locks which
+behave similarly to binary semaphores, and were introduced in 2006[1]
+as an alternative to these. This new data structure provided a number
+of advantages, including simpler interfaces, and at that time smaller
+code (see Disadvantages).
- - 'struct mutex' is smaller on most architectures: E.g. on x86,
- 'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
- A smaller structure size means less RAM footprint, and better
- CPU-cache utilization.
+[1] http://lwn.net/Articles/164802/
- - tighter code. On x86 i get the following .text sizes when
- switching all mutex-alike semaphores in the kernel to the mutex
- subsystem:
+Implementation
+--------------
- text data bss dec hex filename
- 3280380 868188 396860 4545428 455b94 vmlinux-semaphore
- 3255329 865296 396732 4517357 44eded vmlinux-mutex
+Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h
+and implemented in kernel/locking/mutex.c. These locks use a three
+state atomic counter (->count) to represent the different possible
+transitions that can occur during the lifetime of a lock:
- that's 25051 bytes of code saved, or a 0.76% win - off the hottest
- codepaths of the kernel. (The .data savings are 2892 bytes, or 0.33%)
- Smaller code means better icache footprint, which is one of the
- major optimization goals in the Linux kernel currently.
+ 1: unlocked
+ 0: locked, no waiters
+ negative: locked, with potential waiters
- - the mutex subsystem is slightly faster and has better scalability for
- contended workloads. On an 8-way x86 system, running a mutex-based
- kernel and testing creat+unlink+close (of separate, per-task files)
- in /tmp with 16 parallel tasks, the average number of ops/sec is:
+In its most basic form it also includes a wait-queue and a spinlock
+that serializes access to it. CONFIG_SMP systems can also include
+a pointer to the lock task owner (->owner) as well as a spinner MCS
+lock (->osq), both described below in (ii).
- Semaphores: Mutexes:
+When acquiring a mutex, there are three possible paths that can be
+taken, depending on the state of the lock:
- $ ./test-mutex V 16 10 $ ./test-mutex V 16 10
- 8 CPUs, running 16 tasks. 8 CPUs, running 16 tasks.
- checking VFS performance. checking VFS performance.
- avg loops/sec: 34713 avg loops/sec: 84153
- CPU utilization: 63% CPU utilization: 22%
+(i) fastpath: tries to atomically acquire the lock by decrementing the
+ counter. If it was already taken by another task it goes to the next
+ possible path. This logic is architecture specific. On x86-64, the
+ locking fastpath is 2 instructions:
- i.e. in this workload, the mutex based kernel was 2.4 times faster
- than the semaphore based kernel, _and_ it also had 2.8 times less CPU
- utilization. (In terms of 'ops per CPU cycle', the semaphore kernel
- performed 551 ops/sec per 1% of CPU time used, while the mutex kernel
- performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times
- more efficient.)
-
- the scalability difference is visible even on a 2-way P4 HT box:
-
- Semaphores: Mutexes:
-
- $ ./test-mutex V 16 10 $ ./test-mutex V 16 10
- 4 CPUs, running 16 tasks. 8 CPUs, running 16 tasks.
- checking VFS performance. checking VFS performance.
- avg loops/sec: 127659 avg loops/sec: 181082
- CPU utilization: 100% CPU utilization: 34%
-
- (the straight performance advantage of mutexes is 41%, the per-cycle
- efficiency of mutexes is 4.1 times better.)
-
- - there are no fastpath tradeoffs, the mutex fastpath is just as tight
- as the semaphore fastpath. On x86, the locking fastpath is 2
- instructions:
-
- c0377ccb <mutex_lock>:
- c0377ccb: f0 ff 08 lock decl (%eax)
- c0377cce: 78 0e js c0377cde <.text..lock.mutex>
- c0377cd0: c3 ret
+ 0000000000000e10 <mutex_lock>:
+ e21: f0 ff 0b lock decl (%rbx)
+ e24: 79 08 jns e2e <mutex_lock+0x1e>
the unlocking fastpath is equally tight:
- c0377cd1 <mutex_unlock>:
- c0377cd1: f0 ff 00 lock incl (%eax)
- c0377cd4: 7e 0f jle c0377ce5 <.text..lock.mutex+0x7>
- c0377cd6: c3 ret
-
- - 'struct mutex' semantics are well-defined and are enforced if
- CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have
- virtually no debugging code or instrumentation. The mutex subsystem
- checks and enforces the following rules:
-
- * - only one task can hold the mutex at a time
- * - only the owner can unlock the mutex
- * - multiple unlocks are not permitted
- * - recursive locking is not permitted
- * - a mutex object must be initialized via the API
- * - a mutex object must not be initialized via memset or copying
- * - task may not exit with mutex held
- * - memory areas where held locks reside must not be freed
- * - held mutexes must not be reinitialized
- * - mutexes may not be used in hardware or software interrupt
- * contexts such as tasklets and timers
-
- furthermore, there are also convenience features in the debugging
- code:
-
- * - uses symbolic names of mutexes, whenever they are printed in debug output
- * - point-of-acquire tracking, symbolic lookup of function names
- * - list of all locks held in the system, printout of them
- * - owner tracking
- * - detects self-recursing locks and prints out all relevant info
- * - detects multi-task circular deadlocks and prints out all affected
- * locks and tasks (and only those tasks)
+ 0000000000000bc0 <mutex_unlock>:
+ bc8: f0 ff 07 lock incl (%rdi)
+ bcb: 7f 0a jg bd7 <mutex_unlock+0x17>
+
+
+(ii) midpath: aka optimistic spinning, tries to spin for acquisition
+ while the lock owner is running and there are no other tasks ready
+ to run that have higher priority (need_resched). The rationale is
+ that if the lock owner is running, it is likely to release the lock
+ soon. The mutex spinners are queued up using MCS lock so that only
+ one spinner can compete for the mutex.
+
+ The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spinlock
+ with the desirable properties of being fair and with each cpu trying
+ to acquire the lock spinning on a local variable. It avoids expensive
+ cacheline bouncing that common test-and-set spinlock implementations
+ incur. An MCS-like lock is specially tailored for optimistic spinning
+ for sleeping lock implementation. An important feature of the customized
+ MCS lock is that it has the extra property that spinners are able to exit
+ the MCS spinlock queue when they need to reschedule. This further helps
+ avoid situations where MCS spinners that need to reschedule would continue
+ waiting to spin on mutex owner, only to go directly to slowpath upon
+ obtaining the MCS lock.
+
+
+(iii) slowpath: last resort, if the lock is still unable to be acquired,
+ the task is added to the wait-queue and sleeps until woken up by the
+ unlock path. Under normal circumstances it blocks as TASK_UNINTERRUPTIBLE.
+
+While formally kernel mutexes are sleepable locks, it is path (ii) that
+makes them more practically a hybrid type. By simply not interrupting a
+task and busy-waiting for a few cycles instead of immediately sleeping,
+the performance of this lock has been seen to significantly improve a
+number of workloads. Note that this technique is also used for rw-semaphores.
+
+Semantics
+---------
+
+The mutex subsystem checks and enforces the following rules:
+
+ - Only one task can hold the mutex at a time.
+ - Only the owner can unlock the mutex.
+ - Multiple unlocks are not permitted.
+ - Recursive locking/unlocking is not permitted.
+ - A mutex must only be initialized via the API (see below).
+ - A task may not exit with a mutex held.
+ - Memory areas where held locks reside must not be freed.
+ - Held mutexes must not be reinitialized.
+ - Mutexes may not be used in hardware or software interrupt
+ contexts such as tasklets and timers.
+
+These semantics are fully enforced when CONFIG DEBUG_MUTEXES is enabled.
+In addition, the mutex debugging code also implements a number of other
+features that make lock debugging easier and faster:
+
+ - Uses symbolic names of mutexes, whenever they are printed
+ in debug output.
+ - Point-of-acquire tracking, symbolic lookup of function names,
+ list of all locks held in the system, printout of them.
+ - Owner tracking.
+ - Detects self-recursing locks and prints out all relevant info.
+ - Detects multi-task circular deadlocks and prints out all affected
+ locks and tasks (and only those tasks).
+
+
+Interfaces
+----------
+Statically define the mutex:
+ DEFINE_MUTEX(name);
+
+Dynamically initialize the mutex:
+ mutex_init(mutex);
+
+Acquire the mutex, uninterruptible:
+ void mutex_lock(struct mutex *lock);
+ void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
+ int mutex_trylock(struct mutex *lock);
+
+Acquire the mutex, interruptible:
+ int mutex_lock_interruptible_nested(struct mutex *lock,
+ unsigned int subclass);
+ int mutex_lock_interruptible(struct mutex *lock);
+
+Acquire the mutex, interruptible, if dec to 0:
+ int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
+
+Unlock the mutex:
+ void mutex_unlock(struct mutex *lock);
+
+Test if the mutex is taken:
+ int mutex_is_locked(struct mutex *lock);
Disadvantages
-------------
-The stricter mutex API means you cannot use mutexes the same way you
-can use semaphores: e.g. they cannot be used from an interrupt context,
-nor can they be unlocked from a different context that which acquired
-it. [ I'm not aware of any other (e.g. performance) disadvantages from
-using mutexes at the moment, please let me know if you find any. ]
-
-Implementation of mutexes
--------------------------
-
-'struct mutex' is the new mutex type, defined in include/linux/mutex.h and
-implemented in kernel/locking/mutex.c. It is a counter-based mutex with a
-spinlock and a wait-list. The counter has 3 states: 1 for "unlocked", 0 for
-"locked" and negative numbers (usually -1) for "locked, potential waiters
-queued".
-
-the APIs of 'struct mutex' have been streamlined:
-
- DEFINE_MUTEX(name);
+Unlike its original design and purpose, 'struct mutex' is larger than
+most locks in the kernel. E.g: on x86-64 it is 40 bytes, almost twice
+as large as 'struct semaphore' (24 bytes) and 8 bytes shy of the
+'struct rw_semaphore' variant. Larger structure sizes mean more CPU
+cache and memory footprint.
- mutex_init(mutex);
+When to use mutexes
+-------------------
- void mutex_lock(struct mutex *lock);
- int mutex_lock_interruptible(struct mutex *lock);
- int mutex_trylock(struct mutex *lock);
- void mutex_unlock(struct mutex *lock);
- int mutex_is_locked(struct mutex *lock);
- void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
- int mutex_lock_interruptible_nested(struct mutex *lock,
- unsigned int subclass);
- int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
+Unless the strict semantics of mutexes are unsuitable and/or the critical
+region prevents the lock from being shared, always prefer them to any other
+locking primitive.
select MODULES_USE_ELF_RELA if X86_64
select CLONE_BACKWARDS if X86_32
select ARCH_USE_BUILTIN_BSWAP
+ select ARCH_USE_QUEUE_RWLOCK
select OLD_SIGSUSPEND3 if X86_32 || IA32_EMULATION
select OLD_SIGACTION if X86_32
select COMPAT_OLD_SIGACTION if IA32_EMULATION
--- /dev/null
+#ifndef _ASM_X86_QRWLOCK_H
+#define _ASM_X86_QRWLOCK_H
+
+#include <asm-generic/qrwlock_types.h>
+
+#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
+
+#include <asm-generic/qrwlock.h>
+
+#endif /* _ASM_X86_QRWLOCK_H */
cpu_relax();
}
+#ifndef CONFIG_QUEUE_RWLOCK
/*
* Read-write spinlocks, allowing multiple readers
* but only one writer.
asm volatile(LOCK_PREFIX WRITE_LOCK_ADD(%1) "%0"
: "+m" (rw->write) : "i" (RW_LOCK_BIAS) : "memory");
}
+#else
+#include <asm/qrwlock.h>
+#endif /* CONFIG_QUEUE_RWLOCK */
#define arch_read_lock_flags(lock, flags) arch_read_lock(lock)
#define arch_write_lock_flags(lock, flags) arch_write_lock(lock)
#define __ARCH_SPIN_LOCK_UNLOCKED { { 0 } }
+#ifdef CONFIG_QUEUE_RWLOCK
+#include <asm-generic/qrwlock_types.h>
+#else
#include <asm/rwlock.h>
+#endif
#endif /* _ASM_X86_SPINLOCK_TYPES_H */
--- /dev/null
+/*
+ * 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.long@hp.com>
+ */
+#ifndef __ASM_GENERIC_QRWLOCK_H
+#define __ASM_GENERIC_QRWLOCK_H
+
+#include <linux/atomic.h>
+#include <asm/barrier.h>
+#include <asm/processor.h>
+
+#include <asm-generic/qrwlock_types.h>
+
+/*
+ * 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
+ */
+static 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
+ */
+static 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
+ */
+static 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
+ */
+static inline int queue_write_trylock(struct qrwlock *lock)
+{
+ u32 cnts;
+
+ cnts = atomic_read(&lock->cnts);
+ if (unlikely(cnts))
+ return 0;
+
+ return likely(atomic_cmpxchg(&lock->cnts,
+ cnts, cnts | _QW_LOCKED) == cnts);
+}
+/**
+ * queue_read_lock - acquire read lock of a queue rwlock
+ * @lock: Pointer to queue rwlock structure
+ */
+static 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
+ */
+static 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
+ */
+static inline void queue_read_unlock(struct qrwlock *lock)
+{
+ /*
+ * Atomically decrement the reader count
+ */
+ smp_mb__before_atomic();
+ 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
+ */
+static 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();
+ atomic_sub(_QW_LOCKED, &lock->cnts);
+}
+#endif
+
+/*
+ * 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 */
--- /dev/null
+#ifndef __ASM_GENERIC_QRWLOCK_TYPES_H
+#define __ASM_GENERIC_QRWLOCK_TYPES_H
+
+#include <linux/types.h>
+#include <asm/spinlock_types.h>
+
+/*
+ * The queue read/write lock data structure
+ */
+
+typedef struct qrwlock {
+ atomic_t cnts;
+ arch_spinlock_t lock;
+} arch_rwlock_t;
+
+#define __ARCH_RW_LOCK_UNLOCKED { \
+ .cnts = ATOMIC_INIT(0), \
+ .lock = __ARCH_SPIN_LOCK_UNLOCKED, \
+}
+
+#endif /* __ASM_GENERIC_QRWLOCK_TYPES_H */
#include <linux/atomic.h>
+struct optimistic_spin_queue;
struct rw_semaphore;
#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
#else
/* All arch specific implementations share the same struct */
struct rw_semaphore {
- long count;
- raw_spinlock_t wait_lock;
- struct list_head wait_list;
+ long count;
+ raw_spinlock_t wait_lock;
+ struct list_head wait_list;
+#ifdef CONFIG_SMP
+ /*
+ * Write owner. Used as a speculative check to see
+ * if the owner is running on the cpu.
+ */
+ struct task_struct *owner;
+ struct optimistic_spin_queue *osq; /* spinner MCS lock */
+#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
# define __RWSEM_DEP_MAP_INIT(lockname)
#endif
+#if defined(CONFIG_SMP) && !defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
+#define __RWSEM_INITIALIZER(name) \
+ { RWSEM_UNLOCKED_VALUE, \
+ __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock), \
+ LIST_HEAD_INIT((name).wait_list), \
+ NULL, /* owner */ \
+ NULL /* mcs lock */ \
+ __RWSEM_DEP_MAP_INIT(name) }
+#else
#define __RWSEM_INITIALIZER(name) \
{ RWSEM_UNLOCKED_VALUE, \
__RAW_SPIN_LOCK_UNLOCKED(name.wait_lock), \
LIST_HEAD_INIT((name).wait_list) \
__RWSEM_DEP_MAP_INIT(name) }
+#endif
#define DECLARE_RWSEM(name) \
struct rw_semaphore name = __RWSEM_INITIALIZER(name)
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
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
obj-$(CONFIG_LOCK_TORTURE_TEST) += locktorture.o
--- /dev/null
+/*
+ * 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.long@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 <asm/qrwlock.h>
+
+/**
+ * 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)
+{
+ u32 cnts;
+
+ /*
+ * Readers come here when they cannot get the lock without waiting
+ */
+ if (unlikely(in_interrupt())) {
+ /*
+ * 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
+ */
+ arch_spin_lock(&lock->lock);
+
+ /*
+ * 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
+ */
+ arch_spin_unlock(&lock->lock);
+}
+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)
+{
+ u32 cnts;
+
+ /* Put the writer into the wait queue */
+ arch_spin_lock(&lock->lock);
+
+ /* 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.
+ */
+ for (;;) {
+ cnts = atomic_read(&lock->cnts);
+ if (!(cnts & _QW_WMASK) &&
+ (atomic_cmpxchg(&lock->cnts, cnts,
+ cnts | _QW_WAITING) == cnts))
+ break;
+
+ arch_mutex_cpu_relax();
+ }
+
+ /* When no more readers, set the locked flag */
+ for (;;) {
+ cnts = atomic_read(&lock->cnts);
+ if ((cnts == _QW_WAITING) &&
+ (atomic_cmpxchg(&lock->cnts, _QW_WAITING,
+ _QW_LOCKED) == _QW_WAITING))
+ break;
+
+ arch_mutex_cpu_relax();
+ }
+unlock:
+ arch_spin_unlock(&lock->lock);
+}
+EXPORT_SYMBOL(queue_write_lock_slowpath);
*
* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
* and Michel Lespinasse <walken@google.com>
+ *
+ * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
+ * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
*/
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/export.h>
+#include <linux/sched/rt.h>
+
+#include "mcs_spinlock.h"
/*
* Guide to the rw_semaphore's count field for common values.
sem->count = RWSEM_UNLOCKED_VALUE;
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->wait_list);
+#ifdef CONFIG_SMP
+ sem->owner = NULL;
+ sem->osq = NULL;
+#endif
}
EXPORT_SYMBOL(__init_rwsem);
}
/*
- * wait for the read lock to be granted
+ * Wait for the read lock to be granted
*/
__visible
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
return sem;
}
+static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
+{
+ if (!(count & RWSEM_ACTIVE_MASK)) {
+ /* try acquiring the write lock */
+ if (sem->count == RWSEM_WAITING_BIAS &&
+ cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
+ RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
+ if (!list_is_singular(&sem->wait_list))
+ rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
+ return true;
+ }
+ }
+ return false;
+}
+
+#ifdef CONFIG_SMP
/*
- * wait until we successfully acquire the write lock
+ * Try to acquire write lock before the writer has been put on wait queue.
+ */
+static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
+{
+ long old, count = ACCESS_ONCE(sem->count);
+
+ while (true) {
+ if (!(count == 0 || count == RWSEM_WAITING_BIAS))
+ return false;
+
+ old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
+ if (old == count)
+ return true;
+
+ count = old;
+ }
+}
+
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
+{
+ struct task_struct *owner;
+ bool on_cpu = true;
+
+ if (need_resched())
+ return 0;
+
+ rcu_read_lock();
+ owner = ACCESS_ONCE(sem->owner);
+ if (owner)
+ on_cpu = owner->on_cpu;
+ rcu_read_unlock();
+
+ /*
+ * If sem->owner is not set, the rwsem owner may have
+ * just acquired it and not set the owner yet or the rwsem
+ * has been released.
+ */
+ return on_cpu;
+}
+
+static inline bool owner_running(struct rw_semaphore *sem,
+ struct task_struct *owner)
+{
+ if (sem->owner != owner)
+ return false;
+
+ /*
+ * Ensure we emit the owner->on_cpu, dereference _after_ checking
+ * sem->owner still matches owner, if that fails, owner might
+ * point to free()d memory, if it still matches, the rcu_read_lock()
+ * ensures the memory stays valid.
+ */
+ barrier();
+
+ return owner->on_cpu;
+}
+
+static noinline
+bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
+{
+ rcu_read_lock();
+ while (owner_running(sem, owner)) {
+ if (need_resched())
+ break;
+
+ arch_mutex_cpu_relax();
+ }
+ rcu_read_unlock();
+
+ /*
+ * We break out the loop above on need_resched() or when the
+ * owner changed, which is a sign for heavy contention. Return
+ * success only when sem->owner is NULL.
+ */
+ return sem->owner == NULL;
+}
+
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+{
+ struct task_struct *owner;
+ bool taken = false;
+
+ preempt_disable();
+
+ /* sem->wait_lock should not be held when doing optimistic spinning */
+ if (!rwsem_can_spin_on_owner(sem))
+ goto done;
+
+ if (!osq_lock(&sem->osq))
+ goto done;
+
+ while (true) {
+ owner = ACCESS_ONCE(sem->owner);
+ if (owner && !rwsem_spin_on_owner(sem, owner))
+ break;
+
+ /* wait_lock will be acquired if write_lock is obtained */
+ if (rwsem_try_write_lock_unqueued(sem)) {
+ taken = true;
+ break;
+ }
+
+ /*
+ * When there's no owner, we might have preempted between the
+ * owner acquiring the lock and setting the owner field. If
+ * we're an RT task that will live-lock because we won't let
+ * the owner complete.
+ */
+ if (!owner && (need_resched() || rt_task(current)))
+ break;
+
+ /*
+ * The cpu_relax() call is a compiler barrier which forces
+ * everything in this loop to be re-loaded. We don't need
+ * memory barriers as we'll eventually observe the right
+ * values at the cost of a few extra spins.
+ */
+ arch_mutex_cpu_relax();
+ }
+ osq_unlock(&sem->osq);
+done:
+ preempt_enable();
+ return taken;
+}
+
+#else
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+{
+ return false;
+}
+#endif
+
+/*
+ * Wait until we successfully acquire the write lock
*/
__visible
struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
{
- long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS;
+ long count;
+ bool waiting = true; /* any queued threads before us */
struct rwsem_waiter waiter;
- struct task_struct *tsk = current;
- /* set up my own style of waitqueue */
- waiter.task = tsk;
+ /* undo write bias from down_write operation, stop active locking */
+ count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
+
+ /* do optimistic spinning and steal lock if possible */
+ if (rwsem_optimistic_spin(sem))
+ return sem;
+
+ /*
+ * Optimistic spinning failed, proceed to the slowpath
+ * and block until we can acquire the sem.
+ */
+ waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
raw_spin_lock_irq(&sem->wait_lock);
+
+ /* account for this before adding a new element to the list */
if (list_empty(&sem->wait_list))
- adjustment += RWSEM_WAITING_BIAS;
+ waiting = false;
+
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
- count = rwsem_atomic_update(adjustment, sem);
+ if (waiting) {
+ count = ACCESS_ONCE(sem->count);
- /* If there were already threads queued before us and there are no
- * active writers, the lock must be read owned; so we try to wake
- * any read locks that were queued ahead of us. */
- if (count > RWSEM_WAITING_BIAS &&
- adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
- sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
+ /*
+ * If there were already threads queued before us and there are
+ * no active writers, the lock must be read owned; so we try to
+ * wake any read locks that were queued ahead of us.
+ */
+ if (count > RWSEM_WAITING_BIAS)
+ sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
+
+ } else
+ count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
/* wait until we successfully acquire the lock */
- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ set_current_state(TASK_UNINTERRUPTIBLE);
while (true) {
- if (!(count & RWSEM_ACTIVE_MASK)) {
- /* Try acquiring the write lock. */
- count = RWSEM_ACTIVE_WRITE_BIAS;
- if (!list_is_singular(&sem->wait_list))
- count += RWSEM_WAITING_BIAS;
-
- if (sem->count == RWSEM_WAITING_BIAS &&
- cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
- RWSEM_WAITING_BIAS)
- break;
- }
-
+ if (rwsem_try_write_lock(count, sem))
+ break;
raw_spin_unlock_irq(&sem->wait_lock);
/* Block until there are no active lockers. */
do {
schedule();
- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ set_current_state(TASK_UNINTERRUPTIBLE);
} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
raw_spin_lock_irq(&sem->wait_lock);
}
+ __set_current_state(TASK_RUNNING);
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
- tsk->state = TASK_RUNNING;
return sem;
}
#include <linux/atomic.h>
+#if defined(CONFIG_SMP) && defined(CONFIG_RWSEM_XCHGADD_ALGORITHM)
+static inline void rwsem_set_owner(struct rw_semaphore *sem)
+{
+ sem->owner = current;
+}
+
+static inline void rwsem_clear_owner(struct rw_semaphore *sem)
+{
+ sem->owner = NULL;
+}
+
+#else
+static inline void rwsem_set_owner(struct rw_semaphore *sem)
+{
+}
+
+static inline void rwsem_clear_owner(struct rw_semaphore *sem)
+{
+}
+#endif
+
/*
* lock for reading
*/
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
+ rwsem_set_owner(sem);
}
EXPORT_SYMBOL(down_write);
{
int ret = __down_write_trylock(sem);
- if (ret == 1)
+ if (ret == 1) {
rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
+ rwsem_set_owner(sem);
+ }
+
return ret;
}
{
rwsem_release(&sem->dep_map, 1, _RET_IP_);
+ rwsem_clear_owner(sem);
__up_write(sem);
}
* lockdep: a downgraded write will live on as a write
* dependency.
*/
+ rwsem_clear_owner(sem);
__downgrade_write(sem);
}
rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
+ rwsem_set_owner(sem);
}
EXPORT_SYMBOL(_down_write_nest_lock);
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
+ rwsem_set_owner(sem);
}
EXPORT_SYMBOL(down_write_nested);
projects / linux-drm-fsl-dcu.git / commitdiff