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* [RFC PATCH] rcu: Make 'rcu_assign_pointer(p, v)' of type 'typeof(p)'
@ 2019-05-23 13:32 Andrea Parri
  2019-05-23 13:50 ` Paul E. McKenney
  0 siblings, 1 reply; 6+ messages in thread
From: Andrea Parri @ 2019-05-23 13:32 UTC (permalink / raw)
  To: linux-kernel
  Cc: Andrea Parri, Paul E. McKenney, Josh Triplett, Steven Rostedt,
	Mathieu Desnoyers, Lai Jiangshan, Joel Fernandes, rcu,
	Peter Zijlstra, Will Deacon, Mark Rutland

The expression

  rcu_assign_pointer(p, typeof(p) v)

is reported to be of type 'typeof(p)' in the documentation (c.f., e.g.,
Documentation/RCU/whatisRCU.txt) but this is not the case: for example,
the following snippet

  int **y;
  int *x;
  int *r0;


  r0 = rcu_assign_pointer(*y, x);

can currently result in the compiler warning

  warning: assignment to ‘int *’ from ‘uintptr_t’ {aka ‘long unsigned int’} makes pointer from integer without a cast [-Wint-conversion]

Cast the uintptr_t value to a typeof(p) value.

Signed-off-by: Andrea Parri <>
Cc: "Paul E. McKenney" <>
Cc: Josh Triplett <>
Cc: Steven Rostedt <>
Cc: Mathieu Desnoyers <>
Cc: Lai Jiangshan <>
Cc: Joel Fernandes <>
Cc: Peter Zijlstra <>
Cc: Will Deacon <>
Cc: Mark Rutland <>

TBH, I'm not sure this is 'the right patch' (hence the RFC...): in
fact, I'm currently missing the motivations for allowing assignments
such as the "r0 = ..." assignment above in generic code.  (BTW, it's
not currently possible to use such assignments in litmus tests...)

The usual concern is, of course, that if something is allowed (read
'compile!' ;/) then people will soon or later use it and they'll do
it in all sorts of 'creative' ways, such as 'to extend dependencies
across rcu_assign_pointer() calls' as in

  x = READ_ONCE(*z);
  r0 = rcu_assign_pointer(*y, x);
  WRITE_ONCE(*w, r0);

Notice that using a 'do { ... } while (0)', say, would prevent such
tricks/rvalues. (The same approach is used by smp_store_release().)

For a related discussion, please see:


 include/linux/rcupdate.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 915460ec08722..b94ba5de78fba 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -375,7 +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;							      \
+	((typeof(p))_r_a_p__v);						      \

^ permalink raw reply related	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2019-05-23 15:23 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-05-23 13:32 [RFC PATCH] rcu: Make 'rcu_assign_pointer(p, v)' of type 'typeof(p)' Andrea Parri
2019-05-23 13:50 ` Paul E. McKenney
2019-05-23 14:19   ` Mark Rutland
2019-05-23 14:54     ` Andrea Parri
2019-05-23 15:23     ` Paul E. McKenney
2019-05-23 14:53   ` Andrea Parri

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