srcu: Make SRCU readers use ->srcu_ctrs for counter selection

This commit causes SRCU readers to use ->srcu_ctrs for counter
selection instead of ->srcu_idx.  This takes another step towards
array-indexing-free SRCU readers.

[ paulmck: Apply kernel test robot feedback. ]

Co-developed-by: Z qiang <qiang.zhang1211@gmail.com>
Signed-off-by: Z qiang <qiang.zhang1211@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: kernel test robot <oliver.sang@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: paulmckrcu

include/linux/srcutree.h

@@ -101,6 +101,7 @@ struct srcu_usage {
  */
 struct srcu_struct {
 	unsigned int srcu_idx;			/* Current rdr array element. */
+	struct srcu_ctr __percpu *srcu_ctrp;
 	struct srcu_data __percpu *sda;		/* Per-CPU srcu_data array. */
 	struct lockdep_map dep_map;
 	struct srcu_usage *srcu_sup;		/* Update-side data. */
@@ -167,6 +168,7 @@ struct srcu_struct {
 #define __SRCU_STRUCT_INIT(name, usage_name, pcpu_name)						\
 {												\
 	.sda = &pcpu_name,									\
+	.srcu_ctrp = &pcpu_name.srcu_ctrs[0],							\
 	__SRCU_STRUCT_INIT_COMMON(name, usage_name)						\
 }
 
@@ -222,13 +224,12 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf);
  */
 static inline int __srcu_read_lock_lite(struct srcu_struct *ssp)
 {
-	int idx;
+	struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
 
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_lite().");
-	idx = READ_ONCE(ssp->srcu_idx) & 0x1;
-	this_cpu_inc(ssp->sda->srcu_ctrs[idx].srcu_locks.counter); /* Y */
+	this_cpu_inc(scp->srcu_locks.counter); /* Y */
 	barrier(); /* Avoid leaking the critical section. */
-	return idx;
+	return scp - &ssp->sda->srcu_ctrs[0];
 }
 
 /*

kernel/rcu/srcutree.c

@@ -253,8 +253,10 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);
 	INIT_DELAYED_WORK(&ssp->srcu_sup->work, process_srcu);
 	ssp->srcu_sup->sda_is_static = is_static;
-	if (!is_static)
+	if (!is_static) {
 		ssp->sda = alloc_percpu(struct srcu_data);
+		ssp->srcu_ctrp = &ssp->sda->srcu_ctrs[0];
+	}
 	if (!ssp->sda)
 		goto err_free_sup;
 	init_srcu_struct_data(ssp);
@@ -742,12 +744,11 @@ EXPORT_SYMBOL_GPL(__srcu_check_read_flavor);
  */
 int __srcu_read_lock(struct srcu_struct *ssp)
 {
-	int idx;
+	struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
 
-	idx = READ_ONCE(ssp->srcu_idx) & 0x1;
-	this_cpu_inc(ssp->sda->srcu_ctrs[idx].srcu_locks.counter);
+	this_cpu_inc(scp->srcu_locks.counter);
 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
-	return idx;
+	return scp - &ssp->sda->srcu_ctrs[0];
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);
 
@@ -772,13 +773,12 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
  */
 int __srcu_read_lock_nmisafe(struct srcu_struct *ssp)
 {
-	int idx;
-	struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
+	struct srcu_ctr __percpu *scpp = READ_ONCE(ssp->srcu_ctrp);
+	struct srcu_ctr *scp = raw_cpu_ptr(scpp);
 
-	idx = READ_ONCE(ssp->srcu_idx) & 0x1;
-	atomic_long_inc(&sdp->srcu_ctrs[idx].srcu_locks);
+	atomic_long_inc(&scp->srcu_locks);
 	smp_mb__after_atomic(); /* B */  /* Avoid leaking the critical section. */
-	return idx;
+	return scpp - &ssp->sda->srcu_ctrs[0];
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock_nmisafe);
 
@@ -1152,6 +1152,8 @@ static void srcu_flip(struct srcu_struct *ssp)
 	smp_mb(); /* E */  /* Pairs with B and C. */
 
 	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); // Flip the counter.
+	WRITE_ONCE(ssp->srcu_ctrp,
+		   &ssp->sda->srcu_ctrs[!(ssp->srcu_ctrp - &ssp->sda->srcu_ctrs[0])]);
 
 	/*
 	 * Ensure that if the updater misses an __srcu_read_unlock()
@@ -2000,6 +2002,7 @@ static int srcu_module_coming(struct module *mod)
 		ssp->sda = alloc_percpu(struct srcu_data);
 		if (WARN_ON_ONCE(!ssp->sda))
 			return -ENOMEM;
+		ssp->srcu_ctrp = &ssp->sda->srcu_ctrs[0];
 	}
 	return 0;
 }