documentation: Correct doc to use rcu_dereference_protected

As there is lots of misinformation and outdated information on the
Internet about nearly all topics related to the kernel, I thought it
would be best if I based my RCU code on the guidelines of the examples
in the Documentation/ tree of the latest kernel. One thing that stuck
out when reading the whatisRCU.txt document was, "interesting how we
don't need any function to dereference rcu protected pointers when doing
updates if a lock is held. I wonder how static analyzers will work with
that." Then, a few weeks later, upon discovering sparse's __rcu support,
I ran it over my code, and lo and behold, things weren't done right.
Examining other RCU usages in the kernel reveal consistent usage of
rcu_dereference_protected, passing in lockdep_is_held as the
conditional. So, this patch adds that idiom to the documentation, so
that others ahead of me won't endure the same exercise.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index adc2184009c5..dc49c6712b17 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -364,7 +364,7 @@ uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
 	};
 	DEFINE_SPINLOCK(foo_mutex);
 
-	struct foo *gbl_foo;
+	struct foo __rcu *gbl_foo;
 
 	/*
 	 * Create a new struct foo that is the same as the one currently
@@ -386,7 +386,7 @@ uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
 
 		new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
 		spin_lock(&foo_mutex);
-		old_fp = gbl_foo;
+		old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
 		*new_fp = *old_fp;
 		new_fp->a = new_a;
 		rcu_assign_pointer(gbl_foo, new_fp);
@@ -487,7 +487,7 @@ The foo_update_a() function might then be written as follows:
 
 		new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
 		spin_lock(&foo_mutex);
-		old_fp = gbl_foo;
+		old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
 		*new_fp = *old_fp;
 		new_fp->a = new_a;
 		rcu_assign_pointer(gbl_foo, new_fp);