srcu: Add an API for a memory barrier after SRCU read lock

To avoid redundant memory barriers, add smp_mb__after_srcu_read_lock() to
pair with smp_mb__after_srcu_read_unlock() for use in paths that need to
emit a memory barrier, but already do srcu_read_lock(), which includes a
full memory barrier.  Provide an API, e.g. as opposed to having callers
document the behavior via a comment, as the full memory barrier provided
by srcu_read_lock() is an implementation detail that shouldn't bleed into
random subsystems.

KVM will use smp_mb__after_srcu_read_lock() in it's VM-Exit path to ensure
a memory barrier is emitted, which is necessary to ensure correctness of
mixed memory types on CPUs that support self-snoop.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
[sean: massage changelog]
Tested-by: Xiangfei Ma <xiangfeix.ma@intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org
Link: https://lore.kernel.org/r/20240309010929.1403984-4-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

include/linux/srcu.h

@@ -343,6 +343,20 @@ static inline void smp_mb__after_srcu_read_unlock(void)
 	/* __srcu_read_unlock has smp_mb() internally so nothing to do here. */
 }
 
+/**
+ * smp_mb__after_srcu_read_lock - ensure full ordering after srcu_read_lock
+ *
+ * Converts the preceding srcu_read_lock into a two-way memory barrier.
+ *
+ * Call this after srcu_read_lock, to guarantee that all memory operations
+ * that occur after smp_mb__after_srcu_read_lock will appear to happen after
+ * the preceding srcu_read_lock.
+ */
+static inline void smp_mb__after_srcu_read_lock(void)
+{
+	/* __srcu_read_lock has smp_mb() internally so nothing to do here. */
+}
+
 DEFINE_LOCK_GUARD_1(srcu, struct srcu_struct,
 		    _T->idx = srcu_read_lock(_T->lock),
 		    srcu_read_unlock(_T->lock, _T->idx),