srcu: Add SRCU_READ_FLAVOR_SLOWGP to flag need for synchronize_rcu()

This commit switches from a direct test of SRCU_READ_FLAVOR_LITE to a new
SRCU_READ_FLAVOR_SLOWGP macro to check for substituting synchronize_rcu()
for smp_mb() in SRCU grace periods.  Right now, SRCU_READ_FLAVOR_SLOWGP
is exactly SRCU_READ_FLAVOR_LITE, but the addition of the _fast() flavor
of SRCU will change that.

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/srcu.h

@@ -49,6 +49,9 @@ int init_srcu_struct(struct srcu_struct *ssp);
 #define SRCU_READ_FLAVOR_LITE	0x4		// srcu_read_lock_lite().
 #define SRCU_READ_FLAVOR_ALL   (SRCU_READ_FLAVOR_NORMAL | SRCU_READ_FLAVOR_NMI | \
 				SRCU_READ_FLAVOR_LITE) // All of the above.
+#define SRCU_READ_FLAVOR_SLOWGP	SRCU_READ_FLAVOR_LITE
+						// Flavors requiring synchronize_rcu()
+						// instead of smp_mb().
 
 #ifdef CONFIG_TINY_SRCU
 #include <linux/srcutiny.h>

kernel/rcu/srcutree.c

@@ -449,7 +449,7 @@ static bool srcu_readers_lock_idx(struct srcu_struct *ssp, int idx, bool gp, uns
 	}
 	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
 		  "Mixed reader flavors for srcu_struct at %ps.\n", ssp);
-	if (mask & SRCU_READ_FLAVOR_LITE && !gp)
+	if (mask & SRCU_READ_FLAVOR_SLOWGP && !gp)
 		return false;
 	return sum == unlocks;
 }
@@ -487,7 +487,7 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
 	unsigned long unlocks;
 
 	unlocks = srcu_readers_unlock_idx(ssp, idx, &rdm);
-	did_gp = !!(rdm & SRCU_READ_FLAVOR_LITE);
+	did_gp = !!(rdm & SRCU_READ_FLAVOR_SLOWGP);
 
 	/*
 	 * Make sure that a lock is always counted if the corresponding
@@ -1205,7 +1205,7 @@ static bool srcu_should_expedite(struct srcu_struct *ssp)
 
 	check_init_srcu_struct(ssp);
 	/* If _lite() readers, don't do unsolicited expediting. */
-	if (this_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE)
+	if (this_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_SLOWGP)
 		return false;
 	/* If the local srcu_data structure has callbacks, not idle.  */
 	sdp = raw_cpu_ptr(ssp->sda);