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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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Will Deacon 2014-01-21 15:36:10 -08:00 committed by Ingo Molnar
parent 5faeb8adb9
commit e207552e64
1 changed files with 28 additions and 14 deletions
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42
include/linux/mcs_spinlock.h
View File

@ -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 */