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.sho...@oracle.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.
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,
                  void (*func)(struct rcu_head *rcu))
{
    call_rcu(head, func);
}

tree.c:

void kfree_call_rcu(struct rcu_head *head,
            void (*func)(struct rcu_head *rcu))
{
    __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