/* * 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/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> #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 * 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 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;} /* * Trylock variant that retuns the owning task on failure. */static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock){	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);		unsigned long task = owner & ~MUTEX_FLAGS; 		if (task) {			if (likely(task != curr))				break; 			if (likely(!(flags & MUTEX_FLAG_PICKUP)))				break; 			flags &= ~MUTEX_FLAG_PICKUP;		} else {#ifdef CONFIG_DEBUG_MUTEXES			DEBUG_LOCKS_WARN_ON(flags & MUTEX_FLAG_PICKUP);#endif		} 		/*		 * 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.		 */		flags &= ~MUTEX_FLAG_HANDOFF; 		old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);		if (old == owner)			return NULL; 		owner = old;	} 	return __owner_task(owner);} /* * Actual trylock that will work on any unlocked state. */static inline bool __mutex_trylock(struct mutex *lock){	return !__mutex_trylock_or_owner(lock);} #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. Sets PICKUP on a handoff, clears HANDOF, 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 old, new; #ifdef CONFIG_DEBUG_MUTEXES		DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);		DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);#endif 		new = (owner & MUTEX_FLAG_WAITERS);		new |= (unsigned long)task;		if (task)			new |= MUTEX_FLAG_PICKUP; 		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 voidww_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++;} static inline bool __sched__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b){	return a->stamp - b->stamp <= LONG_MAX &&	       (a->stamp != b->stamp || a > b);} /* * Wake up any waiters that may have to back off when the lock is held by the * given context. * * Due to the invariants on the wait list, this can only affect the first * waiter with a context. * * The current task must not be on the wait list. */static void __sched__ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx){	struct mutex_waiter *cur; 	lockdep_assert_held(&lock->wait_lock); 	list_for_each_entry(cur, &lock->wait_list, list) {		if (!cur->ww_ctx)			continue; 		if (cur->ww_ctx->acquired > 0 &&		    __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {			debug_mutex_wake_waiter(lock, cur);			wake_up_process(cur->task);		} 		break;	}} /* * 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){	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(&lock->base.wait_lock);	__ww_mutex_wakeup_for_backoff(&lock->base, ctx);	spin_unlock(&lock->base.wait_lock);} /* * After acquiring lock in the slowpath set ctx. * * Unlike for the fast path, the caller ensures that waiters are woken up where * necessary. * * Callers must hold the mutex wait_lock. */static __always_inline voidww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx){	ww_mutex_lock_acquired(lock, ctx);	lock->ctx = ctx;} #ifdef CONFIG_MUTEX_SPIN_ON_OWNER 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; 	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;		} 		if (ww_ctx && !ww_mutex_spin_on_owner(lock, ww_ctx, waiter)) {			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

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