srcu: Pull pointer-to-integer conversion into __srcu_ptr_to_ctr()

This commit abstracts the srcu_read_lock*() pointer-to-integer conversion
into a new __srcu_ptr_to_ctr().  This will be used in rcutorture for
testing an srcu_read_lock_fast() that returns a pointer rather than
an integer.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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>

include/linux/srcutree.h

@@ -211,6 +211,13 @@ void synchronize_srcu_expedited(struct srcu_struct *ssp);
 void srcu_barrier(struct srcu_struct *ssp);
 void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf);
 
+// Converts a per-CPU pointer to an ->srcu_ctrs[] array element to that
+// element's index.
+static inline bool __srcu_ptr_to_ctr(struct srcu_struct *ssp, struct srcu_ctr __percpu *scpp)
+{
+	return scpp - &ssp->sda->srcu_ctrs[0];
+}
+
 /*
  * Counts the new reader in the appropriate per-CPU element of the
  * srcu_struct.  Returns an index that must be passed to the matching
@@ -228,7 +235,7 @@ static inline int __srcu_read_lock_lite(struct srcu_struct *ssp)
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_lite().");
 	this_cpu_inc(scp->srcu_locks.counter); /* Y */
 	barrier(); /* Avoid leaking the critical section. */
-	return scp - &ssp->sda->srcu_ctrs[0];
+	return __srcu_ptr_to_ctr(ssp, scp);
 }
 
 /*

kernel/rcu/srcutree.c

@@ -753,7 +753,7 @@ int __srcu_read_lock(struct srcu_struct *ssp)
 
 	this_cpu_inc(scp->srcu_locks.counter);
 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
-	return scp - &ssp->sda->srcu_ctrs[0];
+	return __srcu_ptr_to_ctr(ssp, scp);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);
 
@@ -783,7 +783,7 @@ int __srcu_read_lock_nmisafe(struct srcu_struct *ssp)
 
 	atomic_long_inc(&scp->srcu_locks);
 	smp_mb__after_atomic(); /* B */  /* Avoid leaking the critical section. */
-	return scpp - &ssp->sda->srcu_ctrs[0];
+	return __srcu_ptr_to_ctr(ssp, scpp);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock_nmisafe);