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authorPeter Zijlstra <peterz@infradead.org>2016-04-04 11:57:12 +0300
committerIngo Molnar <mingo@kernel.org>2016-06-14 12:54:27 +0300
commit1f03e8d2919270bd6ef64f39a45ce8df8a9f012a (patch)
tree0cdd5de370212a021d0a1a3439bbc4b0a77fea8b /include/linux/compiler.h
parent245050c287a9176cee9f98109df101909c1eeef4 (diff)
downloadlinux-1f03e8d2919270bd6ef64f39a45ce8df8a9f012a.tar.xz
locking/barriers: Replace smp_cond_acquire() with smp_cond_load_acquire()
This new form allows using hardware assisted waiting. Some hardware (ARM64 and x86) allow monitoring an address for changes, so by providing a pointer we can use this to replace the cpu_relax() with hardware optimized methods in the future. Requested-by: Will Deacon <will.deacon@arm.com> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> 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>
Diffstat (limited to 'include/linux/compiler.h')
-rw-r--r--include/linux/compiler.h25
1 files changed, 19 insertions, 6 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 06f27fd9d760..2bcaedc0f032 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -305,21 +305,34 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
})
/**
- * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
+ * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
+ * @ptr: pointer to the variable to wait on
* @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.
*
+ * Due to C lacking lambda expressions we load the value of *ptr into a
+ * pre-named variable @VAL to be used in @cond.
+ *
* 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)
+#ifndef smp_cond_load_acquire
+#define smp_cond_load_acquire(ptr, cond_expr) ({ \
+ typeof(ptr) __PTR = (ptr); \
+ typeof(*ptr) VAL; \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ cpu_relax(); \
+ } \
+ smp_rmb(); /* ctrl + rmb := acquire */ \
+ VAL; \
+})
+#endif
#endif /* __KERNEL__ */