Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read_unlock

[Michael S. Tsirkin]

> > Paul, could you review this patch please?
> > Documentation/memory-barriers.txt says that unlock has a weaker
> > uni-directional barrier, but in practice srcu_read_unlock calls
> > smp_mb().
> > 
> > Is it OK to rely on this? If not, can I add
> > smp_mb__after_srcu_read_unlock (making it an empty macro for now)
> > so we can avoid an actual extra smp_mb()?
> 
> Please use smp_mb__after_srcu_read_unlock().  After all, it was not
> that long ago that srcu_read_unlock() contained no memory barriers,
> and perhaps some day it won't need to once again.
> 
> 							Thanx, Paul
>

Thanks!
Something like this will be enough?
 
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index c114614..9b058ee 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -237,4 +237,18 @@ static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
 	__srcu_read_unlock(sp, idx);
 }
 
+/**
+ * smp_mb__after_srcu_read_unlock - ensure full ordering after srcu_read_unlock
+ *
+ * Converts the preceding srcu_read_unlock into a two-way memory barrier.
+ *
+ * Call this after srcu_read_unlock, to guarantee that all memory operations
+ * that occur after smp_mb__after_srcu_read_unlock will appear to happen after
+ * the preceding srcu_read_unlock.
+ */
+static inline void smp_mb__after_srcu_read_unlock(void)
+{
+	/* __srcu_read_unlock has smp_mb() internally so nothing to do here. */
+}
+
 #endif
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