linux-kernel - Re: [srcu] Can we suppress sparse warning?

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Message-Id: <20161109185810.GM4127@linux.vnet.ibm.com>
Date:   Wed, 9 Nov 2016 10:58:10 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [srcu] Can we suppress sparse warning?

On Wed, Nov 09, 2016 at 08:36:31PM +0900, Tetsuo Handa wrote:
> Hello.
> 
> When I build
> 
> ---------- test/test.c ----------
> #include <linux/module.h>
> 
> static int __init test_init(void)
> {
>         DEFINE_SRCU(srcu);
>         int idx = srcu_read_lock(&srcu);
>         void *ptr = srcu_dereference(ptr, &srcu);

I am really confused by the above line of code...

>         srcu_read_unlock(&srcu, idx);
>         return -EINVAL;
> }
> 
> module_init(test_init);
> MODULE_LICENSE("GPL");
> ---------- test/test.c ----------
> 
> with C=1 option, I get
> 
>   test/test.c:7:21: error: incompatible types in comparison expression (different address spaces)
> 
> warning from
> 
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -605,7 +605,7 @@ static inline void rcu_preempt_sleep_check(void)
>         /* Dependency order vs. p above. */ \
>         typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
>         RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
> -       rcu_dereference_sparse(p, space); \
> +       rcu_dereference_sparse(p, space); /** this line **/ \
>         ((typeof(*p) __force __kernel *)(________p1)); \
>  })
>  #define __rcu_dereference_protected(p, c, space) \
> 
> . 
> 
> I want to use srcu_dereference() at
> https://lists.01.org/pipermail/kbuild-all/2016-October/026587.html .
> Can we have srcu version of 995f1405610bd844 ("rcu: Suppress sparse
> warnings for rcu_dereference_raw()") ?

Notwithstanding my confusion about what your self-referential
srcu_dereference() is intended to achieve, what happens if you change the
"void *ptr = srcu_dereference(ptr, &srcu);" to add __rcu?

	void __rcu *ptr = srcu_dereference(ptr, &srcu);

Or, more to the point:

	void __rcu *gptr;

	void *lptr = srcu_dereference(ptr, &srcu);

							Thanx, Paul