![Davidlohr Bueso](/assets/img/avatar_default.png)
We have two flavors of the MCS spinlock: standard and cancelable (OSQ). While each one is independent of the other, we currently mix and match them. This patch: - Moves the OSQ code out of mcs_spinlock.h (which only deals with the traditional version) into include/linux/osq_lock.h. No unnecessary code is added to the more global header file, anything locks that make use of OSQ must include it anyway. - Renames mcs_spinlock.c to osq_lock.c. This file only contains osq code. - Introduces a CONFIG_LOCK_SPIN_ON_OWNER in order to only build osq_lock if there is support for it. Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mikulas Patocka <mpatocka@redhat.com> Cc: Waiman Long <Waiman.Long@hp.com> Link: http://lkml.kernel.org/r/1420573509-24774-5-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
112 lines
3.2 KiB
C
112 lines
3.2 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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#include <asm/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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#ifndef arch_mcs_spin_lock_contended
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/*
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* Using smp_load_acquire() provides a memory barrier that ensures
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* subsequent operations happen after the lock is acquired.
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*/
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#define arch_mcs_spin_lock_contended(l) \
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do { \
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while (!(smp_load_acquire(l))) \
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cpu_relax_lowlatency(); \
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} while (0)
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#endif
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#ifndef arch_mcs_spin_unlock_contended
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/*
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* smp_store_release() provides a memory barrier to ensure all
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* operations in the critical section has been completed before
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* unlocking.
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*/
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#define arch_mcs_spin_unlock_contended(l) \
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smp_store_release((l), 1)
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#endif
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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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/*
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* In order to acquire the lock, the caller should declare a local node and
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* pass a reference of the node to this function in addition to the lock.
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* If the lock has already been acquired, then this will proceed to spin
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* on this node->locked until the previous lock holder sets the node->locked
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* in mcs_spin_unlock().
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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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/*
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* Lock acquired, don't need to set node->locked to 1. Threads
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* only spin on its own node->locked value for lock acquisition.
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* However, since this thread can immediately acquire the lock
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* and does not proceed to spin on its own node->locked, this
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* value won't be used. If a debug mode is needed to
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* audit lock status, then set node->locked value here.
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*/
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return;
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}
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ACCESS_ONCE(prev->next) = node;
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/* Wait until the lock holder passes the lock down. */
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arch_mcs_spin_lock_contended(&node->locked);
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}
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/*
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* Releases the lock. The caller should pass in the corresponding node that
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* was used to acquire the lock.
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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 (likely(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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cpu_relax_lowlatency();
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}
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/* Pass lock to next waiter. */
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arch_mcs_spin_unlock_contended(&next->locked);
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}
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#endif /* __LINUX_MCS_SPINLOCK_H */
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