[PATCH rcu 3/3] srcu: Unconditionally record srcu_read_lock_lite() in ->srcu_reader_flavor

[Paul E. McKenney]

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@xxxxxxx>
Closes: https://lore.kernel.org/all/d07e8f4a-d5ff-4c8e-8e61-50db285c57e9@xxxxxxx/
Fixes: c0f08d6b5a61 ("srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()"
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 include/linux/srcu.h     |  8 +-------
 include/linux/srcutiny.h |  3 +++
 include/linux/srcutree.h | 21 +++++++++++++++++++++
 kernel/rcu/srcutree.c    |  6 ++----
 4 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index fff6265818d4f..abaddd7e6ddf7 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
@@ -278,7 +272,7 @@ 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 4d96bbdb45f08..1321da803274d 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 778eb61542e18..490aeecc6bb47 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
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 5991381b44383..ee3cbb8308664 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -711,11 +711,10 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
 }
 EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 
-#ifdef CONFIG_PROVE_RCU
 /*
  * Check for consistent reader flavor.
  */
-void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
+void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
 {
 	int old_read_flavor;
 	struct srcu_data *sdp;
@@ -733,8 +732,7 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
 	}
 	WARN_ONCE(old_read_flavor != read_flavor, "CPU %d old state %d new state %d\n", sdp->cpu, old_read_flavor, read_flavor);
 }
-EXPORT_SYMBOL_GPL(srcu_check_read_flavor);
-#endif /* CONFIG_PROVE_RCU */
+EXPORT_SYMBOL_GPL(__srcu_check_read_flavor);
 
 /*
  * Counts the new reader in the appropriate per-CPU element of the
-- 
2.40.1