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We will need the MCS lock code for doing optimistic spinning for rwsem and queued rwlock. Extracting the MCS code from mutex.c and put into its own file allow us to reuse this code easily. We also inline mcs_spin_lock and mcs_spin_unlock functions for better efficiency. Note that using the smp_load_acquire/smp_store_release pair used in mcs_lock and mcs_unlock is not sufficient to form a full memory barrier across cpus for many architectures (except x86). For applications that absolutely need a full barrier across multiple cpus with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be used after mcs_lock. Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1390347360.3138.63.camel@schen9-DESK Signed-off-by: Ingo Molnar <mingo@kernel.org>
77 lines
2.1 KiB
C
77 lines
2.1 KiB
C
/*
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* MCS lock defines
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*
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* This file contains the main data structure and API definitions of MCS lock.
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*
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* The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
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* with the desirable properties of being fair, and with each cpu trying
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* to acquire the lock spinning on a local variable.
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* It avoids expensive cache bouncings that common test-and-set spin-lock
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* implementations incur.
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*/
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#ifndef __LINUX_MCS_SPINLOCK_H
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#define __LINUX_MCS_SPINLOCK_H
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struct mcs_spinlock {
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struct mcs_spinlock *next;
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int locked; /* 1 if lock acquired */
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};
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/*
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* Note: the smp_load_acquire/smp_store_release pair is not
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* sufficient to form a full memory barrier across
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* cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
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* For applications that need a full barrier across multiple cpus
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* with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
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* used after mcs_lock.
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*/
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static inline
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void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
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{
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struct mcs_spinlock *prev;
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/* Init node */
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node->locked = 0;
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node->next = NULL;
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prev = xchg(lock, node);
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if (likely(prev == NULL)) {
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/* Lock acquired */
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node->locked = 1;
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return;
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}
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ACCESS_ONCE(prev->next) = node;
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/*
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* Wait until the lock holder passes the lock down.
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* Using smp_load_acquire() provides a memory barrier that
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* ensures subsequent operations happen after the lock is acquired.
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*/
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while (!(smp_load_acquire(&node->locked)))
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arch_mutex_cpu_relax();
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}
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static inline
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void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
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{
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struct mcs_spinlock *next = ACCESS_ONCE(node->next);
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if (likely(!next)) {
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/*
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* Release the lock by setting it to NULL
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*/
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if (cmpxchg(lock, node, NULL) == node)
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return;
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/* Wait until the next pointer is set */
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while (!(next = ACCESS_ONCE(node->next)))
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arch_mutex_cpu_relax();
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}
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/*
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* Pass lock to next waiter.
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* smp_store_release() provides a memory barrier to ensure
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* all operations in the critical section has been completed
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* before unlocking.
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*/
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smp_store_release(&next->locked, 1);
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}
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#endif /* __LINUX_MCS_SPINLOCK_H */
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