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43b3f02899
Dave ran into horrible performance on a VM without PARAVIRT_SPINLOCKS set and Linus noted that the test-and-set implementation was retarded. One should spin on the variable with a load, not a RMW. While there, remove 'queued' from the name, as the lock isn't queued at all, but a simple test-and-set. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Reported-by: Dave Chinner <david@fromorbit.com> Tested-by: Dave Chinner <david@fromorbit.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Waiman Long <Waiman.Long@hp.com> Cc: stable@vger.kernel.org # v4.2+ Link: http://lkml.kernel.org/r/20150904152523.GR18673@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
139 lines
4.1 KiB
C
139 lines
4.1 KiB
C
/*
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* Queued spinlock
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
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*
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* Authors: Waiman Long <waiman.long@hp.com>
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*/
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#ifndef __ASM_GENERIC_QSPINLOCK_H
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#define __ASM_GENERIC_QSPINLOCK_H
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#include <asm-generic/qspinlock_types.h>
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/**
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* queued_spin_is_locked - is the spinlock locked?
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* @lock: Pointer to queued spinlock structure
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* Return: 1 if it is locked, 0 otherwise
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*/
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static __always_inline int queued_spin_is_locked(struct qspinlock *lock)
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{
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return atomic_read(&lock->val);
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}
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/**
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* queued_spin_value_unlocked - is the spinlock structure unlocked?
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* @lock: queued spinlock structure
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* Return: 1 if it is unlocked, 0 otherwise
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*
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* N.B. Whenever there are tasks waiting for the lock, it is considered
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* locked wrt the lockref code to avoid lock stealing by the lockref
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* code and change things underneath the lock. This also allows some
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* optimizations to be applied without conflict with lockref.
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*/
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static __always_inline int queued_spin_value_unlocked(struct qspinlock lock)
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{
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return !atomic_read(&lock.val);
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}
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/**
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* queued_spin_is_contended - check if the lock is contended
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* @lock : Pointer to queued spinlock structure
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* Return: 1 if lock contended, 0 otherwise
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*/
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static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
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{
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return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
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}
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/**
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* queued_spin_trylock - try to acquire the queued spinlock
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* @lock : Pointer to queued spinlock structure
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* Return: 1 if lock acquired, 0 if failed
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*/
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static __always_inline int queued_spin_trylock(struct qspinlock *lock)
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{
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if (!atomic_read(&lock->val) &&
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(atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) == 0))
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return 1;
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return 0;
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}
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extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
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/**
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* queued_spin_lock - acquire a queued spinlock
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* @lock: Pointer to queued spinlock structure
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*/
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static __always_inline void queued_spin_lock(struct qspinlock *lock)
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{
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u32 val;
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val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
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if (likely(val == 0))
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return;
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queued_spin_lock_slowpath(lock, val);
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}
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#ifndef queued_spin_unlock
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/**
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* queued_spin_unlock - release a queued spinlock
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* @lock : Pointer to queued spinlock structure
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*/
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static __always_inline void queued_spin_unlock(struct qspinlock *lock)
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{
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/*
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* smp_mb__before_atomic() in order to guarantee release semantics
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*/
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smp_mb__before_atomic_dec();
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atomic_sub(_Q_LOCKED_VAL, &lock->val);
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}
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#endif
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/**
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* queued_spin_unlock_wait - wait until current lock holder releases the lock
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* @lock : Pointer to queued spinlock structure
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*
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* There is a very slight possibility of live-lock if the lockers keep coming
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* and the waiter is just unfortunate enough to not see any unlock state.
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*/
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static inline void queued_spin_unlock_wait(struct qspinlock *lock)
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{
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while (atomic_read(&lock->val) & _Q_LOCKED_MASK)
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cpu_relax();
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}
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#ifndef virt_spin_lock
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static __always_inline bool virt_spin_lock(struct qspinlock *lock)
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{
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return false;
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}
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#endif
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/*
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* Initializier
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*/
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#define __ARCH_SPIN_LOCK_UNLOCKED { ATOMIC_INIT(0) }
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/*
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* Remapping spinlock architecture specific functions to the corresponding
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* queued spinlock functions.
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*/
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#define arch_spin_is_locked(l) queued_spin_is_locked(l)
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#define arch_spin_is_contended(l) queued_spin_is_contended(l)
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#define arch_spin_value_unlocked(l) queued_spin_value_unlocked(l)
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#define arch_spin_lock(l) queued_spin_lock(l)
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#define arch_spin_trylock(l) queued_spin_trylock(l)
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#define arch_spin_unlock(l) queued_spin_unlock(l)
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#define arch_spin_lock_flags(l, f) queued_spin_lock(l)
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#define arch_spin_unlock_wait(l) queued_spin_unlock_wait(l)
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#endif /* __ASM_GENERIC_QSPINLOCK_H */
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