[PATCH v2 09/11] srcu: Add __srcu_clone_read_lock()

[Peter Zijlstra <peterz@infradead.org>]

In order to support carrying an srcu_read_lock() section across fork,
where both the parent and child process will do: srcu_read_unlock(),
it is needed to account for the extra decrement with an extra
increment at fork time.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/srcu.h     |    1 +
 include/linux/srcutiny.h |   10 ++++++++++
 kernel/rcu/srcutree.c    |    5 +++++
 3 files changed, 16 insertions(+)

--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -55,6 +55,7 @@ void call_srcu(struct srcu_struct *ssp,
 		void (*func)(struct rcu_head *head));
 void cleanup_srcu_struct(struct srcu_struct *ssp);
 int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
+void __srcu_clone_read_lock(struct srcu_struct *ssp, int idx);
 void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
 void synchronize_srcu(struct srcu_struct *ssp);
 unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -71,6 +71,16 @@ static inline int __srcu_read_lock(struc
 	return idx;
 }
 
+static inline void __srcu_clone_read_lock(struct srcu_struct *ssp, int idx)
+{
+	int newval;
+
+	preempt_disable();  // Needed for PREEMPT_AUTO
+	newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) + 1;
+	WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
+	preempt_enable();
+}
+
 static inline void synchronize_srcu_expedited(struct srcu_struct *ssp)
 {
 	synchronize_srcu(ssp);
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -720,6 +720,11 @@ int __srcu_read_lock(struct srcu_struct
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);
 
+void __srcu_clone_read_lock(struct srcu_struct *ssp, int idx)
+{
+	this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter);
+}
+
 /*
  * Removes the count for the old reader from the appropriate per-CPU
  * element of the srcu_struct.  Note that this may well be a different