// SPDX-License-Identifier: GPL-2.0-only/* * 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.rst. */#include <linux/mutex.h>#include <linux/ww_mutex.h>#include <linux/sched/signal.h>#include <linux/sched/rt.h>#include <linux/sched/wake_q.h>#include <linux/sched/debug.h>#include <linux/export.h>#include <linux/spinlock.h>#include <linux/interrupt.h>#include <linux/debug_locks.h>#include <linux/osq_lock.h> #define CREATE_TRACE_POINTS#include <trace/events/lock.h> #ifndef CONFIG_PREEMPT_RT#include "mutex.h" #ifdef CONFIG_DEBUG_MUTEXES# define MUTEX_WARN_ON(cond) DEBUG_LOCKS_WARN_ON(cond)#else# define MUTEX_WARN_ON(cond)#endif void__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key){	atomic_long_set(&lock->owner, 0);	raw_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 * at least L1_CACHE_BYTES, 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 * Bit2 indicates handoff has been done and we're waiting for pickup. */#define MUTEX_FLAG_WAITERS	0x01#define MUTEX_FLAG_HANDOFF	0x02#define MUTEX_FLAG_PICKUP	0x04 #define MUTEX_FLAGS		0x07 /* * Internal helper function; C doesn't allow us to hide it :/ * * DO NOT USE (outside of mutex code). */static inline struct task_struct *__mutex_owner(struct mutex *lock){	return (struct task_struct *)(atomic_long_read(&lock->owner) & ~MUTEX_FLAGS);} static inline struct task_struct *__owner_task(unsigned long owner){	return (struct task_struct *)(owner & ~MUTEX_FLAGS);} bool mutex_is_locked(struct mutex *lock){	return __mutex_owner(lock) != NULL;}EXPORT_SYMBOL(mutex_is_locked); static inline unsigned long __owner_flags(unsigned long owner){	return owner & MUTEX_FLAGS;} /* * Returns: __mutex_owner(lock) on failure or NULL on success. */static inline struct task_struct *__mutex_trylock_common(struct mutex *lock, 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 flags = __owner_flags(owner);		unsigned long task = owner & ~MUTEX_FLAGS; 		if (task) {			if (flags & MUTEX_FLAG_PICKUP) {				if (task != curr)					break;				flags &= ~MUTEX_FLAG_PICKUP;			} else if (handoff) {				if (flags & MUTEX_FLAG_HANDOFF)					break;				flags |= MUTEX_FLAG_HANDOFF;			} else {				break;			}		} else {			MUTEX_WARN_ON(flags & (MUTEX_FLAG_HANDOFF | MUTEX_FLAG_PICKUP));			task = curr;		} 		if (atomic_long_try_cmpxchg_acquire(&lock->owner, &owner, task | flags)) {			if (task == curr)				return NULL;			break;		}	} 	return __owner_task(owner);} /* * Trylock or set HANDOFF */static inline bool __mutex_trylock_or_handoff(struct mutex *lock, bool handoff){	return !__mutex_trylock_common(lock, handoff);} /* * Actual trylock that will work on any unlocked state. */static inline bool __mutex_trylock(struct mutex *lock){	return !__mutex_trylock_common(lock, false);} #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;	unsigned long zero = 0UL; 	if (atomic_long_try_cmpxchg_acquire(&lock->owner, &zero, curr))		return true; 	return false;} static __always_inline bool __mutex_unlock_fast(struct mutex *lock){	unsigned long curr = (unsigned long)current; 	return atomic_long_try_cmpxchg_release(&lock->owner, &curr, 0UL);}#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;} /* * Add @waiter to a given location in the lock wait_list and set the * FLAG_WAITERS flag if it's the first waiter. */static void__mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,		   struct list_head *list){	debug_mutex_add_waiter(lock, waiter, current); 	list_add_tail(&waiter->list, list);	if (__mutex_waiter_is_first(lock, waiter))		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);} static void__mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter){	list_del(&waiter->list);	if (likely(list_empty(&lock->wait_list)))		__mutex_clear_flag(lock, MUTEX_FLAGS); 	debug_mutex_remove_waiter(lock, waiter, current);} /* * Give up ownership to a specific task, when @task = NULL, this is equivalent * to a regular unlock. Sets PICKUP on a handoff, clears HANDOFF, preserves * WAITERS. Provides RELEASE semantics like a regular unlock, the * __mutex_trylock() provides a 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 new; 		MUTEX_WARN_ON(__owner_task(owner) != current);		MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP); 		new = (owner & MUTEX_FLAG_WAITERS);		new |= (unsigned long)task;		if (task)			new |= MUTEX_FLAG_PICKUP; 		if (atomic_long_try_cmpxchg_release(&lock->owner, &owner, new))			break;	}} #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 #include "ww_mutex.h" #ifdef CONFIG_MUTEX_SPIN_ON_OWNER /* * Trylock variant that returns the owning task on failure. */static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock){	return __mutex_trylock_common(lock, false);} static inlinebool ww_mutex_spin_on_owner(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,			    struct mutex_waiter *waiter){	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.	 *	 * Check this in every inner iteration because we may	 * be racing against another thread's ww_mutex_lock.	 */	if (ww_ctx->acquired > 0 && READ_ONCE(ww->ctx))		return false; 	/*	 * If we aren't on the wait list yet, cancel the spin	 * if there are waiters. We want  to avoid stealing the	 * lock from a waiter with an earlier stamp, since the	 * other thread may already own a lock that we also	 * need.	 */	if (!waiter && (atomic_long_read(&lock->owner) & MUTEX_FLAG_WAITERS))		return false; 	/*	 * Similarly, stop spinning if we are no longer the	 * first waiter.	 */	if (waiter && !__mutex_waiter_is_first(lock, waiter))		return false; 	return true;} /* * Look out! "owner" is an entirely speculative pointer access and not * reliable. * * "noinline" so that this function shows up on perf profiles. */static noinlinebool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,			 struct ww_acquire_ctx *ww_ctx, struct mutex_waiter *waiter){	bool ret = true; 	lockdep_assert_preemption_disabled(); 	while (__mutex_owner(lock) == owner) {		/*		 * Ensure we emit the owner->on_cpu, dereference _after_		 * checking lock->owner still matches owner. And we already		 * disabled preemption which is equal to the RCU read-side		 * crital section in optimistic spinning code. Thus the		 * task_strcut structure won't go away during the spinning		 * period		 */		barrier(); 		/*		 * Use vcpu_is_preempted to detect lock holder preemption issue.		 */		if (!owner_on_cpu(owner) || need_resched()) {			ret = false;			break;		} 		if (ww_ctx && !ww_mutex_spin_on_owner(lock, ww_ctx, waiter)) {			ret = false;			break;		} 		cpu_relax();	} 	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; 	lockdep_assert_preemption_disabled(); 	if (need_resched())		return 0; 	/*	 * We already disabled preemption which is equal to the RCU read-side	 * crital section in optimistic spinning code. Thus the task_strcut	 * structure won't go away during the spinning period.	 */	owner = __mutex_owner(lock);	if (owner)		retval = owner_on_cpu(owner); 	/*	 * 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 __always_inline boolmutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,		      struct mutex_waiter *waiter){	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; 		/* Try to acquire the mutex... */		owner = __mutex_trylock_or_owner(lock);		if (!owner)			break; 		/*		 * There's an owner, wait for it to either		 * release the lock or go to sleep.		 */		if (!mutex_spin_on_owner(lock, owner, ww_ctx, waiter))			goto fail_unlock; 		/*		 * 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.		 */		cpu_relax();	} 	if (!waiter)		osq_unlock(&lock->osq); 	return true;  fail_unlock:	if (!waiter)		osq_unlock(&lock->osq); fail:	/*	 * If we fell out of the spin path because of need_resched(),

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