[PATCH] rcu: Correct doc to use rcu_dereference_protected

[PATCH] rcu: Correct doc to use rcu_dereference_protected

[Jason A. Donenfeld]

As there is lots of misinformation and outdated information on the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>
---
 Documentation/RCU/whatisRCU.txt | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 5746b0c..b852c10 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);
-- 
2.5.0

Re: [PATCH] rcu: Correct doc to use rcu_dereference_protected

[Paul E. McKenney]

On Tue, Aug 11, 2015 at 02:26:33PM +0200, Jason A. Donenfeld wrote:
> 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>

Good catch, queued for 4.4, thank you!

							Thanx, Paul

> ---
>  Documentation/RCU/whatisRCU.txt | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
> index 5746b0c..b852c10 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);
> -- 
> 2.5.0
>