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

Peter Zijlstra
2024-07-11T11:07:27Z

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