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 As there is lots of misinformation and outdated information on the 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@xxxxxxxxx> 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
