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Commit e207552e authored by Will Deacon's avatar Will Deacon Committed by Ingo Molnar
Browse files

locking/mcs: Allow architectures to hook in to contended paths 



When contended, architectures may be able to reduce the polling overhead
in ways which aren't expressible using a simple relax() primitive.

This patch allows architectures to hook into the mcs_{lock,unlock}
functions for the contended cases only.

Reviewed-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarTim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1390347370.3138.65.camel@schen9-DESK


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 5faeb8ad

include/linux/mcs_spinlock.h

+28 −14
Original line number Diff line number Diff line
@@ -17,6 +17,28 @@ struct mcs_spinlock { 
	int locked; /* 1 if lock acquired */

};



#ifndef arch_mcs_spin_lock_contended

/*

 * Using smp_load_acquire() provides a memory barrier that ensures

 * subsequent operations happen after the lock is acquired.

 */

#define arch_mcs_spin_lock_contended(l)					\

do {									\

	while (!(smp_load_acquire(l)))					\

		arch_mutex_cpu_relax();					\

} while (0)

#endif



#ifndef arch_mcs_spin_unlock_contended

/*

 * smp_store_release() provides a memory barrier to ensure all

 * operations in the critical section has been completed before

 * unlocking.

 */

#define arch_mcs_spin_unlock_contended(l)				\

	smp_store_release((l), 1)

#endif



/*

 * Note: the smp_load_acquire/smp_store_release pair is not

 * sufficient to form a full memory barrier across
@@ -58,13 +80,9 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 
		return;

	}

	ACCESS_ONCE(prev->next) = node;

	/*

	 * Wait until the lock holder passes the lock down.

	 * Using smp_load_acquire() provides a memory barrier that

	 * ensures subsequent operations happen after the lock is acquired.

	 */

	while (!(smp_load_acquire(&node->locked)))

		arch_mutex_cpu_relax();



	/* Wait until the lock holder passes the lock down. */

	arch_mcs_spin_lock_contended(&node->locked);

}



/*
@@ -86,13 +104,9 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 
		while (!(next = ACCESS_ONCE(node->next)))

			arch_mutex_cpu_relax();

	}

	/*

	 * Pass lock to next waiter.

	 * smp_store_release() provides a memory barrier to ensure

	 * all operations in the critical section has been completed

	 * before unlocking.

	 */

	smp_store_release(&next->locked, 1);



	/* Pass lock to next waiter. */

	arch_mcs_spin_unlock_contended(&next->locked);

}



#endif /* __LINUX_MCS_SPINLOCK_H */