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locking, sched: Introduce smp_cond_acquire() and use it

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Introduce smp_cond_acquire() which combines a control dependency and a
read barrier to form acquire semantics.

This primitive has two benefits:

 - it documents control dependencies,
 - its typically cheaper than using smp_load_acquire() in a loop.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2015-10-16 14:39:38 +02:00 • committed by Ingo Molnar
parent 829cf31751
commit b3e0b1b6d8
4 changed files with 20 additions and 10 deletions
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17
include/linux/compiler.h
View file

@ -299,6 +299,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
__u.__val; \
})
/**
* smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
* @cond: boolean expression to wait for
*
* Equivalent to using smp_load_acquire() on the condition variable but employs
* the control dependency of the wait to reduce the barrier on many platforms.
*
* The control dependency provides a LOAD->STORE order, the additional RMB
* provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
* aka. ACQUIRE.
*/
#define smp_cond_acquire(cond) do { \
while (!(cond)) \
cpu_relax(); \
smp_rmb(); /* ctrl + rmb := acquire */ \
} while (0)
#endif /* __KERNEL__ */
#endif /* __ASSEMBLY__ */