Messages in this thread Patch in this message | | | From | Andrea Parri <> | Subject | [RFC PATCH] rcu: Make 'rcu_assign_pointer(p, v)' of type 'typeof(p)' | Date | Thu, 23 May 2019 15:32:20 +0200 |
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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 <andrea.parri@amarulasolutions.com> Cc: "Paul E. McKenney" <paulmck@linux.ibm.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Cc: Joel Fernandes <joel@joelfernandes.org> Cc: rcu@vger.kernel.org Cc: Peter Zijlstra <peterz@infradead.org> Cc: Will Deacon <will.deacon@arm.com> Cc: Mark Rutland <mark.rutland@arm.com> --- NOTE:
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:
https://lkml.kernel.org/r/20190523083013.GA4616@andrea
Thoughts?
Andrea --- 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); \ }) /** -- 2.7.4
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