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Date: Fri, 24 May 2019 06:29:11 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Andrea Parri <andrea.parri@...rulasolutions.com>
Cc: linux-kernel@...r.kernel.org,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Will Deacon <will.deacon@....com>,
Mark Rutland <mark.rutland@....com>,
Matthew Wilcox <willy@...radead.org>,
Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH] rcu: Prevent evaluation of rcu_assign_pointer()
On Fri, May 24, 2019 at 12:36:37PM +0200, Andrea Parri wrote:
> Quoting Paul [1]:
>
> "Given that a quick (and perhaps error-prone) search of the uses
> of rcu_assign_pointer() in v5.1 didn't find a single use of the
> return value, let's please instead change the documentation and
> implementation to eliminate the return value."
>
> [1] https://lkml.kernel.org/r/20190523135013.GL28207@linux.ibm.com
Thank you! A few comments below.
Thanx, Paul
> Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: rcu@...r.kernel.org
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Sasha Levin <sashal@...nel.org>
> ---
> Matthew, Sasha:
>
> The patch is based on -rcu/dev; I took the liberty of applying the
> same change to your #defines in:
>
> tools/testing/radix-tree/linux/rcupdate.h
> tools/include/linux/rcu.h
>
> but I admit that I'm not familiar with their uses: please shout if
> you have any objections with it.
> ---
> Documentation/RCU/whatisRCU.txt | 8 ++++----
> include/linux/rcupdate.h | 5 ++---
> tools/include/linux/rcu.h | 11 +++++++++--
> tools/testing/radix-tree/linux/rcupdate.h | 5 ++++-
> 4 files changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
> index 981651a8b65d2..f99a87b9a88fa 100644
> --- a/Documentation/RCU/whatisRCU.txt
> +++ b/Documentation/RCU/whatisRCU.txt
> @@ -212,7 +212,7 @@ synchronize_rcu()
>
> rcu_assign_pointer()
>
> - typeof(p) rcu_assign_pointer(p, typeof(p) v);
> + rcu_assign_pointer(p, typeof(p) v);
Please add the "void", similar to synchronize_rcu() above.
> Yes, rcu_assign_pointer() -is- implemented as a macro, though it
> would be cool to be able to declare a function in this manner.
> @@ -220,9 +220,9 @@ rcu_assign_pointer()
>
> The updater uses this function to assign a new value to an
> RCU-protected pointer, in order to safely communicate the change
> - in value from the updater to the reader. This function returns
> - the new value, and also executes any memory-barrier instructions
> - required for a given CPU architecture.
> + in value from the updater to the reader. This macro does not
> + evaluate to an rvalue, but it does execute any memory-barrier
> + instructions required for a given CPU architecture.
>
> Perhaps just as important, it serves to document (1) which
> pointers are protected by RCU and (2) the point at which a
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 915460ec08722..a5f61a08e65fc 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -367,7 +367,7 @@ static inline void rcu_preempt_sleep_check(void) { }
> * other macros that it invokes.
> */
> #define rcu_assign_pointer(p, v) \
> -({ \
> +do { \
> uintptr_t _r_a_p__v = (uintptr_t)(v); \
> rcu_check_sparse(p, __rcu); \
> \
> @@ -375,8 +375,7 @@ static inline void rcu_preempt_sleep_check(void) { }
> WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \
> else \
> smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
> - _r_a_p__v; \
> -})
> +} while (0)
>
> /**
> * rcu_swap_protected() - swap an RCU and a regular pointer
> diff --git a/tools/include/linux/rcu.h b/tools/include/linux/rcu.h
> index 7d02527e5bcea..01a435ee48cd6 100644
> --- a/tools/include/linux/rcu.h
> +++ b/tools/include/linux/rcu.h
> @@ -19,7 +19,14 @@ static inline bool rcu_is_watching(void)
> return false;
> }
>
> -#define rcu_assign_pointer(p, v) ((p) = (v))
> -#define RCU_INIT_POINTER(p, v) p=(v)
> +#define rcu_assign_pointer(p, v) \
> +do { \
> + (p) = (v); \
> +} while (0)
> +
> +#define RCU_INIT_POINTER(p, v) \
> +do { \
> + (p) = (v); \
> +} while (0)
These two each fit nicely on one line:
#define rcu_assign_pointer(p, v) do { (p) = (v); } while (0)
#define RCU_INIT_POINTER(p, v) do { (p) = (v); } while (0)
>
> #endif
> diff --git a/tools/testing/radix-tree/linux/rcupdate.h b/tools/testing/radix-tree/linux/rcupdate.h
> index fd280b070fdb1..48212f3a758e6 100644
> --- a/tools/testing/radix-tree/linux/rcupdate.h
> +++ b/tools/testing/radix-tree/linux/rcupdate.h
> @@ -7,6 +7,9 @@
> #define rcu_dereference_raw(p) rcu_dereference(p)
> #define rcu_dereference_protected(p, cond) rcu_dereference(p)
> #define rcu_dereference_check(p, cond) rcu_dereference(p)
> -#define RCU_INIT_POINTER(p, v) (p) = (v)
> +#define RCU_INIT_POINTER(p, v) \
> +do { \
> + (p) = (v); \
> +} while (0)
As does this one.
> #endif
> --
> 2.7.4
>
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