linux-kernel - Re: [PATCH] Move kfree_call_rcu() to slab

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Date:   Thu, 21 Dec 2017 04:36:30 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     rao.shoaib@...cle.com
Cc:     linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
        brouer@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH] Move kfree_call_rcu() to slab_common.c

On Thu, Dec 21, 2017 at 12:19:47AM -0800, rao.shoaib@...cle.com wrote:
> This patch moves kfree_call_rcu() and related macros out of rcu code. A new
> function __call_rcu_lazy() is created for calling __call_rcu() with the lazy
> flag.

Something you probably didn't know ... there are two RCU implementations
in the kernel; Tree and Tiny.  It looks like you've only added
__call_rcu_lazy() to Tree and you'll also need to add it to Tiny.

> Also moving macros generated following checkpatch noise. I do not know
> how to silence checkpatch as there is nothing wrong.
> 
> CHECK: Macro argument reuse 'offset' - possible side-effects?
> #91: FILE: include/linux/slab.h:348:
> +#define __kfree_rcu(head, offset) \
> +	do { \
> +		BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
> +		kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
> +	} while (0)

What checkpatch is warning you about here is that somebody might call

__kfree_rcu(p, a++);

and this would expand into

	do { \
		BUILD_BUG_ON(!__is_kfree_rcu_offset(a++)); \
		kfree_call_rcu(p, (rcu_callback_t)(unsigned long)(a++)); \
	} while (0)

which would increment 'a' twice, and cause pain and suffering.

That's pretty unlikely usage of __kfree_rcu(), but I suppose it's not
impossible.  We have various hacks to get around this kind of thing;
for example I might do this as::

#define __kfree_rcu(head, offset) \
	do { \
		unsigned long __o = offset;
		BUILD_BUG_ON(!__is_kfree_rcu_offset(__o)); \
		kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(__o)); \
	} while (0)

Now offset is only evaluated once per invocation of the macro.  The other
two warnings are the same problem.