srcu: Unconditionally record srcu_read_lock_lite() in ->srcu_reader_flavor

Currently, srcu_read_lock_lite() uses the SRCU_READ_FLAVOR_LITE bit in
->srcu_reader_flavor to communicate to the grace-period processing in
srcu_readers_active_idx_check() that the smp_mb() must be replaced by a
synchronize_rcu().  Unfortunately, ->srcu_reader_flavor is not updated
unless the kernel is built with CONFIG_PROVE_RCU=y.  Therefore in all
kernels built with CONFIG_PROVE_RCU=n, srcu_readers_active_idx_check()
incorrectly uses smp_mb() instead of synchronize_rcu() for srcu_struct
structures whose readers use srcu_read_lock_lite().

This commit therefore causes Tree SRCU srcu_read_lock_lite()
to unconditionally update ->srcu_reader_flavor so that
srcu_readers_active_idx_check() can make the correct choice.

Reported-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Closes: https://lore.kernel.org/all/d07e8f4a-d5ff-4c8e-8e61-50db285c57e9@amd.com/
Fixes: c0f08d6b5a61 ("srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

3 files changed, 25 insertions, 7 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 56f83237de4d..08339eb8a01c 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -183,12 +183,6 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
 
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
-#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_TREE_SRCU)
-void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
-#else
-#define srcu_check_read_flavor(ssp, read_flavor) do { } while (0)
-#endif
-
 
 /**
  * srcu_dereference_check - fetch SRCU-protected pointer for later dereferencing
@@ -277,7 +271,7 @@ static inline int srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp)
 {
 	int retval;
 
-	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE);
+	srcu_check_read_flavor_lite(ssp);
 	retval = __srcu_read_lock_lite(ssp);
 	rcu_try_lock_acquire(&ssp->dep_map);
 	return retval;
diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index 4d96bbdb45f0..1321da803274 100644
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -81,6 +81,9 @@ static inline void srcu_barrier(struct srcu_struct *ssp)
 	synchronize_srcu(ssp);
 }
 
+#define srcu_check_read_flavor(ssp, read_flavor) do { } while (0)
+#define srcu_check_read_flavor_lite(ssp) do { } while (0)
+
 /* Defined here to avoid size increase for non-torture kernels. */
 static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
 					    char *tt, char *tf)
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 778eb61542e1..490aeecc6bb4 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -248,4 +248,25 @@ static inline void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx)
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_lite().");
 }
 
+void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
+
+// Record _lite() usage even for CONFIG_PROVE_RCU=n kernels.
+static inline void srcu_check_read_flavor_lite(struct srcu_struct *ssp)
+{
+	struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
+
+	if (likely(READ_ONCE(sdp->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE))
+		return;
+
+	// Note that the cmpxchg() in srcu_check_read_flavor() is fully ordered.
+	__srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE);
+}
+
+// Record non-_lite() usage only for CONFIG_PROVE_RCU=y kernels.
+static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
+{
+	if (IS_ENABLED(CONFIG_PROVE_RCU))
+		__srcu_check_read_flavor(ssp, read_flavor);
+}
+
 #endif