/* * kernel/locking/mutex.c * * Mutexes: blocking mutual exclusion locks * * Started by Ingo Molnar: * *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> * * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and * David Howells for suggestions and improvements. * *  - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline *    from the -rt tree, where it was originally implemented for rtmutexes *    by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale *    and Sven Dietrich. * * Also see Documentation/locking/mutex-design.txt. */#include <linux/mutex.h>#include <linux/ww_mutex.h>#include <linux/sched.h>#include <linux/sched/rt.h>#include <linux/export.h>#include <linux/spinlock.h>#include <linux/interrupt.h>#include <linux/debug_locks.h>#include <linux/osq_lock.h> #ifdef CONFIG_DEBUG_MUTEXES# include "mutex-debug.h"#else# include "mutex.h"#endif void__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key){	atomic_long_set(&lock->owner, 0);	spin_lock_init(&lock->wait_lock);	INIT_LIST_HEAD(&lock->wait_list);#ifdef CONFIG_MUTEX_SPIN_ON_OWNER	osq_lock_init(&lock->osq);#endif 	debug_mutex_init(lock, name, key);}EXPORT_SYMBOL(__mutex_init); /* * @owner: contains: 'struct task_struct *' to the current lock owner, * NULL means not owned. Since task_struct pointers are aligned at * ARCH_MIN_TASKALIGN (which is at least sizeof(void *)), we have low * bits to store extra state. * * Bit0 indicates a non-empty waiter list; unlock must issue a wakeup. * Bit1 indicates unlock needs to hand the lock to the top-waiter */#define MUTEX_FLAG_WAITERS	0x01#define MUTEX_FLAG_HANDOFF	0x02 #define MUTEX_FLAGS		0x03 static inline struct task_struct *__owner_task(unsigned long owner){	return (struct task_struct *)(owner & ~MUTEX_FLAGS);} static inline unsigned long __owner_flags(unsigned long owner){	return owner & MUTEX_FLAGS;} /* * Actual trylock that will work on any unlocked state. * * When setting the owner field, we must preserve the low flag bits. * * Be careful with @handoff, only set that in a wait-loop (where you set * HANDOFF) to avoid recursive lock attempts. */static inline bool __mutex_trylock(struct mutex *lock, const bool handoff){	unsigned long owner, curr = (unsigned long)current; 	owner = atomic_long_read(&lock->owner);	for (;;) { /* must loop, can race against a flag */		unsigned long old, flags = __owner_flags(owner); 		if (__owner_task(owner)) {			if (handoff && unlikely(__owner_task(owner) == current)) {				/*				 * Provide ACQUIRE semantics for the lock-handoff.				 *				 * We cannot easily use load-acquire here, since				 * the actual load is a failed cmpxchg, which				 * doesn't imply any barriers.				 *				 * Also, this is a fairly unlikely scenario, and				 * this contains the cost.				 */				smp_mb(); /* ACQUIRE */				return true;			} 			return false;		} 		/*		 * We set the HANDOFF bit, we must make sure it doesn't live		 * past the point where we acquire it. This would be possible		 * if we (accidentally) set the bit on an unlocked mutex.		 */		if (handoff)			flags &= ~MUTEX_FLAG_HANDOFF; 		old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);		if (old == owner)			return true; 		owner = old;	}} #ifndef CONFIG_DEBUG_LOCK_ALLOC/* * Lockdep annotations are contained to the slow paths for simplicity. * There is nothing that would stop spreading the lockdep annotations outwards * except more code. */ /* * Optimistic trylock that only works in the uncontended case. Make sure to * follow with a __mutex_trylock() before failing. */static __always_inline bool __mutex_trylock_fast(struct mutex *lock){	unsigned long curr = (unsigned long)current; 	if (!atomic_long_cmpxchg_acquire(&lock->owner, 0UL, curr))		return true; 	return false;} static __always_inline bool __mutex_unlock_fast(struct mutex *lock){	unsigned long curr = (unsigned long)current; 	if (atomic_long_cmpxchg_release(&lock->owner, curr, 0UL) == curr)		return true; 	return false;}#endif static inline void __mutex_set_flag(struct mutex *lock, unsigned long flag){	atomic_long_or(flag, &lock->owner);} static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag){	atomic_long_andnot(flag, &lock->owner);} static inline bool __mutex_waiter_is_first(struct mutex *lock, struct mutex_waiter *waiter){	return list_first_entry(&lock->wait_list, struct mutex_waiter, list) == waiter;} /* * Give up ownership to a specific task, when @task = NULL, this is equivalent * to a regular unlock. Clears HANDOFF, preserves WAITERS. Provides RELEASE * semantics like a regular unlock, the __mutex_trylock() provides matching * ACQUIRE semantics for the handoff. */static void __mutex_handoff(struct mutex *lock, struct task_struct *task){	unsigned long owner = atomic_long_read(&lock->owner); 	for (;;) {		unsigned long old, new; #ifdef CONFIG_DEBUG_MUTEXES		DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);#endif 		new = (owner & MUTEX_FLAG_WAITERS);		new |= (unsigned long)task; 		old = atomic_long_cmpxchg_release(&lock->owner, owner, new);		if (old == owner)			break; 		owner = old;	}} #ifndef CONFIG_DEBUG_LOCK_ALLOC/* * We split the mutex lock/unlock logic into separate fastpath and * slowpath functions, to reduce the register pressure on the fastpath. * We also put the fastpath first in the kernel image, to make sure the * branch is predicted by the CPU as default-untaken. */static void __sched __mutex_lock_slowpath(struct mutex *lock); /** * mutex_lock - acquire the mutex * @lock: the mutex to be acquired * * Lock the mutex exclusively for this task. If the mutex is not * available right now, it will sleep until it can get it. * * The mutex must later on be released by the same task that * acquired it. Recursive locking is not allowed. The task * may not exit without first unlocking the mutex. Also, kernel * memory where the mutex resides must not be freed with * the mutex still locked. The mutex must first be initialized * (or statically defined) before it can be locked. memset()-ing * the mutex to 0 is not allowed. * * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging *   checks that will enforce the restrictions and will also do *   deadlock debugging. ) * * This function is similar to (but not equivalent to) down(). */void __sched mutex_lock(struct mutex *lock){	might_sleep(); 	if (!__mutex_trylock_fast(lock))		__mutex_lock_slowpath(lock);}EXPORT_SYMBOL(mutex_lock);#endif static __always_inline void ww_mutex_lock_acquired(struct ww_mutex *ww,						   struct ww_acquire_ctx *ww_ctx){#ifdef CONFIG_DEBUG_MUTEXES	/*	 * If this WARN_ON triggers, you used ww_mutex_lock to acquire,	 * but released with a normal mutex_unlock in this call.	 *	 * This should never happen, always use ww_mutex_unlock.	 */	DEBUG_LOCKS_WARN_ON(ww->ctx); 	/*	 * Not quite done after calling ww_acquire_done() ?	 */	DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire); 	if (ww_ctx->contending_lock) {		/*		 * After -EDEADLK you tried to		 * acquire a different ww_mutex? Bad!		 */		DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock != ww); 		/*		 * You called ww_mutex_lock after receiving -EDEADLK,		 * but 'forgot' to unlock everything else first?		 */		DEBUG_LOCKS_WARN_ON(ww_ctx->acquired > 0);		ww_ctx->contending_lock = NULL;	} 	/*	 * Naughty, using a different class will lead to undefined behavior!	 */	DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);#endif	ww_ctx->acquired++;} /* * After acquiring lock with fastpath or when we lost out in contested * slowpath, set ctx and wake up any waiters so they can recheck. */static __always_inline voidww_mutex_set_context_fastpath(struct ww_mutex *lock,			       struct ww_acquire_ctx *ctx){	unsigned long flags;	struct mutex_waiter *cur; 	ww_mutex_lock_acquired(lock, ctx); 	lock->ctx = ctx; 	/*	 * The lock->ctx update should be visible on all cores before	 * the atomic read is done, otherwise contended waiters might be	 * missed. The contended waiters will either see ww_ctx == NULL	 * and keep spinning, or it will acquire wait_lock, add itself	 * to waiter list and sleep.	 */	smp_mb(); /* ^^^ */ 	/*	 * Check if lock is contended, if not there is nobody to wake up	 */	if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))		return; 	/*	 * Uh oh, we raced in fastpath, wake up everyone in this case,	 * so they can see the new lock->ctx.	 */	spin_lock_mutex(&lock->base.wait_lock, flags);	list_for_each_entry(cur, &lock->base.wait_list, list) {		debug_mutex_wake_waiter(&lock->base, cur);		wake_up_process(cur->task);	}	spin_unlock_mutex(&lock->base.wait_lock, flags);} /* * After acquiring lock in the slowpath set ctx and wake up any * waiters so they can recheck. * * Callers must hold the mutex wait_lock. */static __always_inline voidww_mutex_set_context_slowpath(struct ww_mutex *lock,			      struct ww_acquire_ctx *ctx){	struct mutex_waiter *cur; 	ww_mutex_lock_acquired(lock, ctx);	lock->ctx = ctx; 	/*	 * Give any possible sleeping processes the chance to wake up,	 * so they can recheck if they have to back off.	 */	list_for_each_entry(cur, &lock->base.wait_list, list) {		debug_mutex_wake_waiter(&lock->base, cur);		wake_up_process(cur->task);	}} #ifdef CONFIG_MUTEX_SPIN_ON_OWNER/* * Look out! "owner" is an entirely speculative pointer * access and not reliable. */static noinlinebool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner){	bool ret = true; 	rcu_read_lock();	while (__mutex_owner(lock) == owner) {		/*		 * Ensure we emit the owner->on_cpu, dereference _after_		 * checking lock->owner still matches owner. If that fails,		 * owner might point to freed memory. If it still matches,		 * the rcu_read_lock() ensures the memory stays valid.		 */		barrier(); 		/*		 * Use vcpu_is_preempted to detect lock holder preemption issue.		 */		if (!owner->on_cpu || need_resched() ||				vcpu_is_preempted(task_cpu(owner))) {			ret = false;			break;		} 		cpu_relax();	}	rcu_read_unlock(); 	return ret;} /* * Initial check for entering the mutex spinning loop */static inline int mutex_can_spin_on_owner(struct mutex *lock){	struct task_struct *owner;	int retval = 1; 	if (need_resched())		return 0; 	rcu_read_lock();	owner = __mutex_owner(lock); 	/*	 * As lock holder preemption issue, we both skip spinning if task is not	 * on cpu or its cpu is preempted	 */	if (owner)		retval = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));	rcu_read_unlock(); 	/*	 * If lock->owner is not set, the mutex has been released. Return true	 * such that we'll trylock in the spin path, which is a faster option	 * than the blocking slow path.	 */	return retval;} /* * Optimistic spinning. * * We try to spin for acquisition when we find that the lock owner * is currently running on a (different) CPU and while we don't * need to reschedule. 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. However, if mutex spinning isn't * going to happen, there is no point in going through the lock/unlock * overhead. * * Returns true when the lock was taken, otherwise false, indicating * that we need to jump to the slowpath and sleep. * * The waiter flag is set to true if the spinner is a waiter in the wait * queue. The waiter-spinner will spin on the lock directly and concurrently * with the spinner at the head of the OSQ, if present, until the owner is * changed to itself. */static bool mutex_optimistic_spin(struct mutex *lock,				  struct ww_acquire_ctx *ww_ctx,				  const bool use_ww_ctx, const bool waiter){	struct task_struct *task = current; 	if (!waiter) {		/*		 * The purpose of the mutex_can_spin_on_owner() function is		 * to eliminate the overhead of osq_lock() and osq_unlock()		 * in case spinning isn't possible. As a waiter-spinner		 * is not going to take OSQ lock anyway, there is no need		 * to call mutex_can_spin_on_owner().		 */		if (!mutex_can_spin_on_owner(lock))			goto fail; 		/*		 * In order to avoid a stampede of mutex spinners trying to		 * acquire the mutex all at once, the spinners need to take a		 * MCS (queued) lock first before spinning on the owner field.		 */		if (!osq_lock(&lock->osq))			goto fail;	} 	for (;;) {		struct task_struct *owner; 		if (use_ww_ctx && ww_ctx->acquired > 0) {			struct ww_mutex *ww; 			ww = container_of(lock, struct ww_mutex, base);			/*			 * If ww->ctx is set the contents are undefined, only			 * by acquiring wait_lock there is a guarantee that			 * they are not invalid when reading.			 *			 * As such, when deadlock detection needs to be			 * performed the optimistic spinning cannot be done.			 */			if (READ_ONCE(ww->ctx))				goto fail_unlock;		} 		/*		 * If there's an owner, wait for it to either		 * release the lock or go to sleep.		 */		owner = __mutex_owner(lock);		if (owner) {			if (waiter && owner == task) {				smp_mb(); /* ACQUIRE */				break;			} 			if (!mutex_spin_on_owner(lock, owner))				goto fail_unlock;		} 		/* Try to acquire the mutex if it is unlocked. */		if (__mutex_trylock(lock, waiter))			break; 		/*		 * The cpu_relax() call is a compiler barrier which forces		 * everything in this loop to be re-loaded. We don't need

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