Re: [PATCH v3 1/3] rcu: Introduce rcu_swap_protected()

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Mon, 2017-08-28 at 14:26 -0700, Paul E. McKenney wrote:
> On Mon, Aug 28, 2017 at 01:46:13PM -0700, Bart Van Assche wrote:
> > A common pattern in RCU code is to assign a new value to an RCU
> > pointer after having read and stored the old value. Introduce a
> > macro for this pattern.
> > 
> > Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Hannes Reinecke <hare@suse.de>
> > Cc: Johannes Thumshirn <jthumshirn@suse.de>
> > Cc: Shane M Seymour <shane.seymour@hpe.com>
> > ---
> >  include/linux/rcupdate.h | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index f816fc72b51e..555815ce2e57 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -561,6 +561,26 @@ static inline void rcu_preempt_sleep_check(void) { }
> >   */
> >  #define rcu_pointer_handoff(p) (p)
> > 
> > +/**
> > + * rcu_swap_protected() - swap an RCU and a regular pointer
> > + * @rcu_ptr: RCU pointer
> > + * @ptr: regular pointer
> > + * @c: the conditions under which the dereference will take place
> > + *
> > + * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
> > + * @c is the argument that is passed to the rcu_dereference_protected() call
> > + * used to read that pointer.
> > + */
> > +#define rcu_swap_protected(rcu_ptr, ptr, c) do {			\
> > +	typeof(ptr) __tmp;						\
> > +									\
> > +	BUILD_BUG_ON(!__same_type(typeof(rcu_ptr), typeof(*(ptr)) __rcu *)); \
> > +	BUILD_BUG_ON(!__same_type(typeof(ptr), typeof(*(rcu_ptr)) *));	\
> 
> Hmmm...
> 
> What kinds of bugs have these two BUILD_BUG_ON() instances have caught
> that would not be caught by the assignments below?

Hello Paul,

These two BUILD_BUG_ON() statements can be left out. The purpose of these
statements is to complain as early as possible if the type of rcu_ptr and/or
ptr is incorrect. As we all know error messages that are triggered by macros
used inside a macro definition can be hard to read. My hope is that these
two BUILD_BUG_ON() macros will cause the compiler to report easier to read
diagnostic messages.

> > +	__tmp = rcu_dereference_protected((rcu_ptr), (c));		\
> > +	rcu_assign_pointer((rcu_ptr), (ptr));				\
> > +	(ptr) = __tmp;							\
> > +} while (0)
> > +
> 
> Could you please put this after rcu_assign_pointer() and before
> rcu_access_pointer()?  That way the things that assign to RCU-protected
> pointers are together.

Something like the patch below (compile-tested only)?

Thanks,

Bart.


[PATCH] rcu: Introduce rcu_swap_protected()
---
 include/linux/rcupdate.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f816fc72b51e..8e920f0ecb07 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -407,6 +407,22 @@ static inline void rcu_preempt_sleep_check(void) { }
 	_r_a_p__v;							      \
 })
 
+/**
+ * rcu_swap_protected() - swap an RCU and a regular pointer
+ * @rcu_ptr: RCU pointer
+ * @ptr: regular pointer
+ * @c: the conditions under which the dereference will take place
+ *
+ * Perform swap(@rcu_ptr, @ptr) where @rcu_ptr is an RCU-annotated pointer and
+ * @c is the argument that is passed to the rcu_dereference_protected() call
+ * used to read that pointer.
+ */
+#define rcu_swap_protected(rcu_ptr, ptr, c) do {			\
+	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
+	rcu_assign_pointer((rcu_ptr), (ptr));				\
+	(ptr) = __tmp;							\
+} while (0)
+
 /**
  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
  * @p: The pointer to read