[PATCH rcu 11/17] srcu: Pull integer-to-pointer conversion into __srcu_ctr_to_ptr()

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

This commit abstracts the srcu_read_unlock*() integer-to-pointer
conversion into a new __srcu_ctr_to_ptr().  This will be used
in rcutorture for testing an srcu_read_unlock_fast() that avoids
array-indexing overhead by taking a pointer rather than an integer.

[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: <bpf@xxxxxxxxxxxxxxx>
---
 include/linux/srcutree.h | 9 ++++++++-
 kernel/rcu/srcutree.c    | 6 ++----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index f41bb3a55a048..55fa400624bb4 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -218,6 +218,13 @@ static inline bool __srcu_ptr_to_ctr(struct srcu_struct *ssp, struct srcu_ctr __
 	return scpp - &ssp->sda->srcu_ctrs[0];
 }
 
+// Converts an integer to a per-CPU pointer to the corresponding
+// ->srcu_ctrs[] array element.
+static inline struct srcu_ctr __percpu *__srcu_ctr_to_ptr(struct srcu_struct *ssp, int idx)
+{
+	return &ssp->sda->srcu_ctrs[idx];
+}
+
 /*
  * Counts the new reader in the appropriate per-CPU element of the
  * srcu_struct.  Returns an index that must be passed to the matching
@@ -252,7 +259,7 @@ static inline int __srcu_read_lock_lite(struct srcu_struct *ssp)
 static inline void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx)
 {
 	barrier();  /* Avoid leaking the critical section. */
-	this_cpu_inc(ssp->sda->srcu_ctrs[idx].srcu_unlocks.counter);  /* Z */
+	this_cpu_inc(__srcu_ctr_to_ptr(ssp, idx)->srcu_unlocks.counter);  /* Z */
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_lite().");
 }
 
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 4643a8ed7e326..d2a6949445538 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -765,7 +765,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
 void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
 {
 	smp_mb(); /* C */  /* Avoid leaking the critical section. */
-	this_cpu_inc(ssp->sda->srcu_ctrs[idx].srcu_unlocks.counter);
+	this_cpu_inc(__srcu_ctr_to_ptr(ssp, idx)->srcu_unlocks.counter);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 
@@ -794,10 +794,8 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock_nmisafe);
  */
 void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
 {
-	struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
-
 	smp_mb__before_atomic(); /* C */  /* Avoid leaking the critical section. */
-	atomic_long_inc(&sdp->srcu_ctrs[idx].srcu_unlocks);
+	atomic_long_inc(&raw_cpu_ptr(__srcu_ctr_to_ptr(ssp, idx))->srcu_unlocks);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock_nmisafe);
 
-- 
2.40.1