Re: [PATCH] Move kfree_call_rcu() to slab_common.c

[Rao Shoaib]

On 12/21/2017 04:36 AM, Matthew Wilcox wrote:
> On Thu, Dec 21, 2017 at 12:19:47AM -0800, rao.shoaib@xxxxxxxxxx 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.
I left it out on purpose because the call in tiny is a little different

rcutiny.h:

static inline void kfree_call_rcu(struct rcu_head *head,
ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â void (*func)(struct rcu_head *rcu))
{
ÂÂÂ call_rcu(head, func);
}

tree.c:

void kfree_call_rcu(struct rcu_head *head,
ÂÂÂ ÂÂÂ ÂÂÂ void (*func)(struct rcu_head *rcu))
{
ÂÂÂ __call_rcu(head, func, rcu_state_p, -1, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);

If we want the code to be exactly same I can create a lazy version for 
tiny as well. However, I don not know where to move kfree_call_rcu() 
from it's current home in rcutiny.h though. Any thoughts ?
> > 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.
Thanks. I was not sure if I was required to fix the noise or based on 
inspection the noise could be ignored. I will make the change and resubmit.

Shoaib